I have a Siamese cat, who will turn 12 years old this year, had his balls chopped off when he was about 6 months old so he has knowledge of sex or any such things. 
But tonight when he was outside I heard a strange noise outside the window and saw this and it surely looks like the other cat is humping my cat! 
This went on for at least 2 minutes, I know he can easily fight off cats that big but I trimmed his claws yesterday so he might not have been able to defend himself, or that’s what I hope happened. 
He hasn’t turned gay has he?