One of the highest compliments I receive is when a man - in particular a gay man - tells me the sex scenes are "hot" and realistic. Often I am asked how they come to be that way.
Let me first say that I enjoy writing them. Our sexual relationships reflect who we are at our core; what happens in the bedroom reflects our inner lives in and out of the bedroom. For that reason, when I am telling an emotional story about humans evolving I can't tell it without including that very important part of human experience. I can honestly say that, although my sex scenes are sometimes detailed, I have never written one solely to titillate: to me that puts one in the realm of pornography. Rather, I write them because they are integral to the story of a character's experience and evolution in the course of the book.
As to how a sex scene between men comes to be true to life, I have little explanation. I can tell you that I do research. I read - about fellatio from a man's point of view, about anal sex and gay men, about male experience. I have watched gay porn - but most of it is emotionless and stale. I read some gay porn; that sometimes gives me an idea or two. But after giving this some thought I have to say that the most important ingredient is that I envy men their sexual experience. I think that men take ownership over and enjoy their sexual experience far more - in general - than women do.
Part of the reason behind this is cultural: men are taught to be sexual and are considered healthy, normal and virile when they do. A woman who is overtly sexual is referred to negatively. Another reason is the inherent reality of pregnancy: sexual behavior is a two-edged sword for women - it almost never comes without physical and emotional risk.
But whatever the reasons, men behave differently than women, and as a writer I find that this has to come into play. I am a little surprised and disturbed by "gay romance" I have read where the roles of the two lovers are little different than that of heterosexual lovers in a Harlequin romance: the formula is the same, and one of them plays a decidedly submissive (translate: feminine) role. I don't find this realistic - and if the author of such a book has as her/his goal mere fantasy and entertainment that is fine. For me personally though, I always grapple with the feeling that I want to treat the community about which I am writing respectfully, and to that end I try to get inside men's heads as much as I can. The ability of men - heterosexual or homosexual - to grab sex by the horns, to behave as if lust and enjoyment are their birthright, is amazing and enviable to me.
The sheer unapologetic joy with which men seem to approach sex is inspiring to me. I believe that when writing a sex scene, I start from that joy and sense of entitlement, and write the scene around it. So far, it seems to be working.