Yesterday my boyfriend and I were fooling around. We did some mutual masturbation, and he ejaculated on my leggings. When I put them back on, they were still a bit wet on the back of my thigh.
I've read that sperm dies after a few minutes outside of the body, and that it cannot travel through the fabric of my underwear.
My period is late (only by one day, but still) and I'm super stressed out.
Is there any chance I could be pregnant? What do I do?
There's the key word right there. If yesterday was the day you were fooling around, and yesterday was the day your period was supposed to occur, there's hardly any chance at all you are pregnant today from that little bit of tomfoolery, and it certainly won't be the reason your period is late. Given your question though: sperm can live for several hours at room temperature: couple that with the above facts and question, and it seems your education on these things is lacking.
So here's some free advice: Get a good book on pregnancy from the library and read it, and next time you fool around with your boyfriend, use protection. Consider this situation a wake up call, I suppose.