Imagine you are with a girl you just met, when was the last time she had cum in her? Was it last week? Was it last night? Was it this morning? Was it two hours ago? You have no idea if you are going to be slurping up her grool or some dude's seed.
Imagine you and your long-term girlfriend had sex this morning, does she still have cum in her at 8pm? Maybe you had sex last night, does she have cum in her the next morning? How about 8pm? When is it "safe" to go down on her without cumming in your own mouth via a 3rd party?
It depends on the person, just like not every girl knows what their discharge taste like.
Some guys get curious, some guys would never(gross, homophobia, no interest), some guys involuntary do/have(surprise snowball, bad aim, she had a prior transaction).
There is a big difference between being curious once, avoiding eating residual creampie, and enjoying eating a cummy puss.