Lube after sex

It's the latter. Your colon doesn't do much more than simply hold waste before it exits via the anus. Anything in there is going to get expelled eventually.

Also, if you're using lube designed for sex specifically it's not going to harm you since it is intended to go into a human body. Ignoring M2M sex for a moment, M/F couples typically use the same kind of stuff and I've yet to hear of a rash of women getting sick because their vaginas got lubed up.
 
I would assume it depends on the lube. Water based...I suppose it would be absorbed in part. Your colon is designed to pull moisture as it holds waste. But I doubt it would be unsafe.

A silicone based lube? Oh that gets expelled, lol