Male cat mounts female cat

My male cat is 3 years old and my female is 2 years old, both fixed. They get along pretty well, are playful, bathe each other and sometimes snuggle. The thing is, whenever me and my female cat are snuggling under the blankets, my male cat jumps on the bed and bites her neck and starts humping her (over the blanket). She ends up getting pretty irritated, growls and sometimes hisses and hits him before leaving the bed altogether. Then he stops. Other times he just stomps on her over the blanket until she gets angry and leaves. And I am wondering, is it jealousy/territorial behaviour or sexual urges? I think it’s also important to say that he is imprinted on me and wants to be close all the time. Nevertheless, he accepted our female cat from the get-go and never showed signs of jealousy.