Sperm competition as a reason why cuckolding excites me so much?

I'd like to discuss a theory I've been reading about for a while: Could it be that the arousal caused by cuckolding is at least partly explained by evolutionary mechanisms such as sperm competition?

For me, few things excite me as much as the thought of my girlfriend having sex with another man and especially the idea of him cumming inside her, filling her up with his sperm. From a rational point of view, this should make me jealous, but emotionally it's the exact opposite: it makes me so horny, my dick gets rock hard and I want to fuck her so badly

In evolutionary biology, there is the concept of sperm competition between sperm from different men to fertilise an egg. Some theories suggest that men might be sexually aroused in certain situations when they feel they are in a ‘competition’ to pass on their genes. But then the question, why are not all men into cuckolding?