Hi. I Was Currently Messing Around With My Boyfriend. He

Hi. I was currently messing around with my boyfriend. He is telling me not to freak out, but I clearly am. So what happened was we ended up doing anal - because I don't want to get pregnant by having sex of course. I know the chances of getting pregnant are none unless it leaks into your vaginal opening. None leaked down to mine but about an hour after we did that I went to use the restroom and some of it, still in my butt (I guess), leaked down into my underwear as I was taking them off. I looked at my underwear and they were wet. I could not tell if it was from me or from him. I also could not tell if it got onto my vagina. If it was his and it did get on my vagina, can I get pregnant from that even though it was about an hour later? Was the sperm living or dead?