I'd probably start off with an age-appropriate book first. I think most of them talk a little about same-sex relationships, but not sure how graphic they get.
Tbh I think its better to talk about more general sexual behaviour first - don't make it just about penetration. Gay men do other things apart from "willies up bottoms" (I believe, not being one myself )
Ds1 is just 10 and I bought him the Usborne book of "what is happening to me" purely because he had started to smell sweaty and unwashed after a normal day, ie no sporting activity that day. He was 9.
That covers puberty and sex. It also covers feelings, emotions, girls and what they go through puberty wise, this may be the best start to it all.
Re gay men, I know several and they don't engage in anal sex, in fact I believe that statistically that heterosexual couples engage in anal sex more than homosexuals do
I don't think you have to specifically mention oral sex, possibly talk about cuddling, hand holding, etc, physical contact feeling nice and that is all part of sex. It isn't just about making babies and willies in vaginas.
I sometimes think that if you haven't had time to prepare an answer, you are like a rabbit in headlights. My Dh is certainly like this. The Usborne book is really good for age appropriate ways to put things across.