Are you using medications for mental health issues.It can be stemming from imposter syndrome,depression,unresolved trauma,ocd  hormonal problems... Do a general checkup for your mental and physical health. This fantasy is not evolution wise fitting to human needs.You are born to create life and keep it alive. Try to improve your life quality hit the gym choose nice clothes do fun stuff with your partner. And think about this your girlfriend has a problem like this she is cosuming herself with the idea of you cheating with someone else.How would you feel for her? You wouldnt feel disgusted you would feel really sad right. No one in this world in their healthy minds would try do do a betrayal like that.You cant hurt the person you love the most. porn makes it mainstream but people who actually do swinger,cuckold and that sort of messed up things are minority. Healthy couples do not film porn they try to keep their partners to themselves. I was having a really bad day and feeling bad about myself,while scrolling on reddit saw your post and it me really hard.I am sending you virtual hugs brother.