Straight guys enjoy seeing that they pleasured their partner. Its self validation that they can make you feel good. Some might also be turned on from a girls doing that, ie a fetish.
It may be similar to why a female feels good if a guy is erect for her. It is also something they can see and believe isn't faked. It's a visual confirmation that the woman is turned on.
I think it's just the general "men love anything sexual to do with vaginas". Especially considering the fact that it only happens when their sexually aroused, and the fact that it is one of the few visual indicators women have of arousal.
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