There can often be a feeling, when writing in a sex scene, that one is doing it just to have it there.  One of the pleasures of writing porn is that a sex scene is never wholly gratuitous, because this is porn, and sex is part of the reason people read it.  You have a duty to add in more sex if you can.

But some scenes feel more gratuitous than others, as they don't seem to advance the plot any.  It seems odd to say, but many fuck scenes do in fact advance the plot of a story, and there is a difference between those, and the ones you include just because you can.  I take an almost visceral pride in those, because this is porn, and it's fun to say "screw you" to the rules of pacing and plot density and add in more fucking just because you can.

Writing same sex scenes always feels slightly dishonest to me, because I cannot say I have ever had it - I've never been with a man, and I've never been a woman.  I have to extrapolate based on being a man and having been with women many times, and I always hope I do okay.  One unique problem with such scenes is pronouns - everybody gets the same one, so editing the text can be challenging.  You don't want to use names as signifiers every single line, but you need to be clear who is doing what.  In a hetero scene, when you say "she" everyone knows who you mean.  In a lesbian scene it can rapidly get confusing, and I imagine the characters stopping to say "Wait, which one of us just did that?"