You occasionally see intercrural sex in fics, but I don't think I've ever seen intergluteal sex, and really, with Matt's ass, that's something that could be pulled off.

Bonus points for Matt getting frustrated at the lack of penetration going on.
"Dammit Foggy, so help me, if you don't stick your cock in me right now, I swear I will ninja you onto your back and do it myself!"