Warning for sex (especially explicit sex vs. some foreplay and fade-to-black) is appreciated and appropriate.
Description of the sex involved, with as many descriptive bits as the people around seem to be in need of, including who is having the sex, what kind of sex they are having, and the gender, sex, or both, of all involved, is also appreciated and appropriate.
An adult (adult-by-behavior, specifically) can look at a description and summary and decide whether or not, by the description, this particular thing is for them.
Basically, when you warn for same-sex sexyfuntimes, and state that it's a warning, you're not just providing a neutral description of the contents. You are explicitly saying, "I am buying into the meme that same-sex sexyfuntimes is alarming and bad!" even though you are writing it and you know that people will read it. You are catering to the concept that people who are squicked by it are totally right to be squicked by it and a lot of people are squicked by it and we don't blame you for being squicked by it, it's nasty gay sex.
Some people are squicked by it, and if they don't want to read it, that's their business; as long as they don't dispute my right to have sex in any way I choose to with any other consenting adult or adults, nor the right of my virtual brother and his partner to have sex (though really, I could do without the details there; virtual family doesn't make them less family -- this is one instance of same-sex smut that I really really don't need to read), nor the right of any other adults to have delightful vanilla, kinky, smutty, consensual sexy funtimes -- then that's their business whether they seek it out or avoid it.
But, for fuck's sake, I will not cater to people who declare that gay sex needs more warnings than straight sex, nor do I even want to appear to cater to them by allowing a warning about sex (that happens to be gay) to read as if it were a warning for OMG GAY sex.
If you happen to do this, I will probably not speak to you personally about it, because most practically, I will either forget to do it, forget who I've already reminded and entirely not remind some people at all, and remind others ten times, or (alas) only remember when something else annoys me, like when I just don't like somebody's icon or something. And that would be wankier than I want to actually be. And goodness knows that there are probably some warnings like that in my back catalog. But going forward, I don't want to keep perpetuating this meme. And if you do see me do it, poke me.