Why do guys do this?

So Everytime a go to have sex with someone new I make it a personal rule of mine no going down on me. I'm fine with basically what ever just don't do that yet. Except Everytime I say this they always ask if I'm sure. I say yes I'm pretty sure but they go and do it anyway. I'll tell them to stop but they just keep going. Even though it feels good it's like really I just fucking said no I didn't want to do that yet. My reasoning I'm not a 100% comfortable with you so doing that it will only make me nervous. So the whole time theyre down there doing their thing I just fake it like I like it even though mentally I'm sweating bullets. So why do all the guys do this? Every one of them I've been with does this. Should I just get rid of the rule because they'll do what ever they want anyway?