So show us the “biological basis.”
The literature is voluminous. There is no way I can post a half file drawer of scientific literature here. But let me try to approach your question in a way that I hope might shed some light on my thinking without getting too technical.
Male homosexuality is not an isolated trait; with it come a variety of characteristics. If you were asked to name the trait most likely to identify a man as homosexual, I'd guess you would cite the sibilant lisp that has become part of the gay stereotype. In fact, you would be right. I suppose you might also name certain occupational preferences, such as interior decorating and hairstyling. Also accurate observations. But there are less recognized traits also. As a group, gay men have a cognitive pattern that trends toward that of females: on average, higher verbal but poorer targeting ability than heterosexual males. In fact, research has found gay males are not significantly better than females at accurately hitting a target (think of throwing an object at a bullseye). From an early age, they also have a history of sex-atypical behavior (e.g. playing with dolls).
You can argue that all of this is learned, but IMO it gets ridiculous when you have so many traits to account for and so little evidence that anyone teaches males to do any of these things. All social conditioning of males is in the opposite direction; with the exception of feminist moonbats, no one encourages males to play with dolls, style hair, and be lousy at things like throwing. Or consider the lisp. How plausible is it that gay men learn to lisp after they decide they are homosexual? So much ridicule accompanies the lisp that one must ask why anyone would voluntarily adopt it. Even more problematic are reports of gay men who had the lisp in elementary school. It's illogical to argue that they learned this lisp as young children in preparation for becoming gay in their late teens. And how would gay men in the first study of targeting ability have known that they should not outperform the females? No one could have taught them that, because no one knew that this characteristic was associated with homosexuality.
I am sure you are familiar with the concept known as Occam's razor: the simplest explanation for a phenomenon is the preferred explanation. Here, the simplest explanation is that some aspect of the prenatal environment affects the brain of the developing fetus in a feminizing manner. As the child develops, his ability pattern and personal preferences trend toward those of the female (hence the preference for male sex partners). If this is the case, the causal factor(s) remain to be identified. For some time, there was interest in the relative balance of sex hormones in the prenatal environment, but I think the weight of the evidence is now against this. The most promising research going on right now is probably work looking at effects from the mother's immune response.
This is not to say that all instances of male homosexuality have the same cause or even that all instances have a biological basis. But I do think that biological influence is involved in the story. Twin studies are also relevant to that conclusion, but this is getting long and I have other things I must get done.
I hope this has been helpful.