Aaron, most women will do that because of two things: insecurity and curiosity. Insecurity because of the orifice the penis was just inside of and what is commonly encountered in there, not wanting the guy to smell the ‘real thin’ which honestly as a guy, I’m aware of the smell and it doesn’t bother me at all. Regarding the curiosity, women would hate to admit it, but they’re aroused by the smell of their ass and what better opportunity to have a whiff than to smell their boyfriend’s penis which was seconds ago inside their rectum! The majority of the time, it doesn’t smell anywhere close to what they’re expecting, in fact, it’s mostly inoffensive for the most part. Guys, just do the right thing if your female partner does this and let her know that her sucking your penis after it was inside her ass is the hottest thing you’ve ever experienced. That should ease her insecurities and possibly help her do it again at a later date.