Why do I keep getting off on the idea of my girlfriend with other men? Whats wrong with me?

Hello everyone

I hope this is a judgement free zone. This is something that is quite embarrassing for me to talk about and I would really appreciate support.

Im a M21. Ive been dating my current girlfriend for about two years. In the past year and a half Ive gotten super into the idea of her sleeping with other men, specifically with bigger penises than me. Often times it turns me on way more than even having sex with her.

I masturbate to the thought of it at least once daily and Its really starting to eat me up now. I feel like such a loser and a joke of a man. Im debating if I should quit watching porn and just start going to church a whole bunch.

I just feel like such a loser talking about this because I dont understand why I feel this way and I feel like no one can relate.

I hope to hear from some of you, and look forward to hopefully getting some advice on this.