Warning - minirant ahead.
When writing, PLEASE take a minute to construct the sex scene in your head before you commit it to paper. Draw it out. Shoot, act it out. Whatever. If you don't have a willing partner, find a couple of dolls or, heck, Care Bears even (I am resisting reading that
yuletide fic).
If person A is on his back, then person B is going to have a difficult time entering "from behind and licking down his spine". While some characters may be fairly limber, this would be quite the challenge for even the most flexible.
It only takes a touch of planning to get your boys or girls or whoever into positions that don't leave your reader trying to picture a pretzel having sex with Gumby.
End minirant.
Thank you.