Not at all selfish. For me I love being bred and if it's a public venue you're probably not going to be the last one who breeds me (I hope!) I can get off later thinking about the loads in my ass.
I agree with BigBart. Whilst it may seem selfish on my part, when I am breeding a willing ass, it's my orgasm that counts. And I am sure that most bottoms would prefer it if the Top shot his load inside their asses first.