OK, I'm going to be contrarian for a moment: the best clothes for sex consist of four layers, ankle to chin, and they're all bulky to boot. Make it hard to remove, make it take ages. Lots of belts, lots of buttons, lots of zippers.
Because that's paragraphs and paragraphs of potentially good stuff, talking about every inch of skin that's slowly being revealed, about the anticipation building and building and building... All that is great shit that you deny yourself and your readers if you reduce it to "neither of them were wearing underwear so he stuck it right in".
P.S. Careful with those zippers, though.