I agree with others, anal sex is better with a condom anyways for more lube and of course more hygienic, you can be the cleanest of person and still have bad smells, you can buy the flavoured condoms which helps with the smell as well as giving oral sex with no rubber taste in your mouth!
Yeah, it's probably just the fact it's sex, I'm not very experienced..(only had it once and probably messed up badly)..but sweat stinks, combine that with semen, your partner's bum, and that's a recipe for some kind of smell. =D
-Jag
I know this is probably a little late but I hope it's helpful.
My boyfriend and I have had anal sex a number of times and I've never noticed too bad a smell coming from his penis. This probably sounds nasty and all but I don't believe we've ever used a condom doing anal. So if I haven't noticed a smell coming from my boyfriend, then I highly doubt you have too much to worry about. Either your partner had a really rank dirty anus or you just have a stanky penis, at the time. I don't think you have anything to worry about. lol Think about it, you're penis ain't gonna smell like roses after sex, whether vaginal or anal. :P
Relax, you had protected sex and you didn't have a risk.