What Causes Blood In Semen Of A Homosexual?

I was with my long term partner last night and we are a gay couple I usually am the one who plays the male role so to speak and we do sometimes have unprotected sex with each other but we are in a completely monogamous relationship as I ejaculated I notice that one part of my semen that shot out had blood in it this was only one small part tthough today I have tried ejaculating again but there was no blood today what could be the cause ?