From an atheist's perspective, (any atheists can feel free to correct me if I'm mistaken about this) sex is essentially a biological function (a necessary one for the propagation of the species, and sometimes even considered vital for the individual’s continued wellbeing). Its purpose is for procreation, but it is largely used recreationally. It is associated with (but not inextricably tied to) love, and is usually most conveniently indulged within a relationship to avoid issues of jealousy or of prolonged unavailability.
The general approach to becoming mature sexually is in experimenting enough to know what you want and how to get it. That a person needs to discover their “type” (that is the kind of person who suits their predetermined desires). For most people this ends up looking like a search for a desired person who is their “type” and a sexual relationship with a person fitting that description a nearly as practicable.
As I am led to understand, an atheist generally believes that a person is sexually "orientated" toward a particular type of sexual experience – possibly as a part of their biological make up, and ought to live according to their “orientation”. This is a belief based largely on what society and popular culture teach.
There is, to my knowledge, thus far no evidence that sexual orientation is in any way scientifically distinguishable beyond the simple observation of a person’s choices and emotional state. Neither is there any reason to believe that a person’s “type” is pre-determined rather than shaped by their experiences.
For a Christian, sex is as much a spiritual act as it is a biological or recreational one. It is designed for the purpose of creating an unbreakable union (that is “one flesh” see Genesis 2:24 and Matthew 19:5f and others) between two people (specifically one of each gender) which must not be broken for any reason barring the death of one of them, or if one of them is behaving in a sexually immoral way (Matthew 5:32 and 19:8).
A Christian marriage is meant to provide a relationship in which both partners seek not to satisfy themselves, but each in selfless love to serve their spouse and take care of their spouse’s needs and desires which are formed by the experiences they have both shared together, and with no basis for external comparison. The question of “is this person your type” or “are they any good” becomes irrelevant because your “type” is based upon your spouse, and your basis for knowing what good sex is like is also based on them. In fact, they are precisely your “type” in a way no one else ever can be, and sex with them is always going to be the best you have ever had.
This creates an environment of love, trust and fidelity into which to bring children who will be raised not just in the presence of two parents, but rather two people who have sexually and emotionally matured enough to keep that union strong for life. The fact that some people fail to live up to the ideal does not in any way diminish the ideal. In the same way that a hundred runners failing to equal the world record doesn’t diminish the record.
Sexual maturity for the Christian is found in putting aside childish base desires and lusts and maintaining fidelity to the union, both before marriage in chastity, and during it in monogamy. Chastity before marriage is so highly valued because by remaining pure before marriage a person demonstrates their commitment to purity and fidelity as training for continued fidelity in marriage. Once married, their undefiled desires are shaped by their first and only partner so that their desire will always be for their spouse. This is also why pornography (and much else in our over sexualised society that is not generally considered to be pornography) is such a huge issue, because young people are exposed to so much sexualised imagery and so many things that pre-shape their “type” so that their eventual spouse has to compete with an impossible fantasy.
So in the Christian framework, a homosexual person (or a philanderer for an alternative example) is one who is sexually immature in that they accept less than the ideal in service to their own desires - they have not mastered their own sexuality and recognised that their "orientation" (or infidelity) is a personal choice formed from experiences rather than a biological imperative. Their desire is one borne out of the licentious society we all live in, and the sometimes tragic and painful events of their past which are in need of healing. Their desire for a union that will emotionally and sexually satisfy them is a selfish corruption of their God-given desire for the ideal one God instituted, but rather than try to deal with their brokenness, they revel in it.
From this framework, both the philanderer and the homosexual (and a number of other folks I haven't mentioned) are in a position of being immature for the purpose of child raising and will likely raise children who will find it exceedingly hard to grow into the maturity that their parents lacked. This is not to say that a person who claims to be a heterosexual Christian faithful to their marriage cannot be an immature person who will raise immature children (as I said, there are a number of other folks I didn't mention) hopefully such a person has a good minister to counsel them before marriage and a good community to support them during the trials life throws at them. The analogy of the blind leading the blind (Matthew 5:14) does seem to apply. I can speak from some experience in this area, as I owe much to the folks in my past who talked me out of several decisions to get married at different times in my life that would likely have led to just an end. I was immature, and am better off now (happily married) for having accepted their advice.
I'm also not going to categorically state that two immature parents can't raise a child who appears to be fairly well socially adjusted. The truth is we're all broken people and some people's brokenness is easier to see than others. Of course your own brokenness tends to be less obvious to you, and your personal favourite sin of choice tends to be less distasteful to you than that of someone else.
I have a few gay friends (admittedly not very many, and I suspect one less since a recent discussion in which I was bagged as a “hater”) and I know some people who, though they had previously been described as gay, are now Christians who have embraced this kind of understanding of sexual maturity and chose to change their “orientation” in order to become sexually mature (in the Christian understanding of it), which they tell me has worked out very well for them. It is interesting, as a side note, that the word orientation generally means “the way you are facing” and repentance literally means to “turn around” in the opposite direction you were previously heading.