Okay so I’m not seeing anywhere in the comments what I think is the right answer so I’ll just give my 2 cents. The reason why it shocks guys is because your “positive” interpretation of consent checking (aka equivalent to you saying “I wanna have sex with youuu!”) is not the general/immediate interpretation. The first thing that generally pops into mind (to everyone, guys and girls) is the negative interpretation - which is the same interpretation that u would get (and I’m gonna exaggerate the language to make it more obvious) if u said “btw, you are not r*ping me, I give consent”. Immediately, assuming you are in cosy environment/ are pretty comfortable with one another, shocks the guy, because all the sudden he is considering the question of “wait, am I raping her?”, “I would never do that!”, “but wait, she is saying that I’m not r*ping her, so that’s okay?”, “but why would she bring the r*pe topic when I’m about to kiss her?”. And it can take an avg well intended guy about 30 seconds all the implications of what u said, and the mood/way the girl said it.
An analogy, which is not perfect I admit, would be like, when the girl and the guy are about to kiss, the guy saying “btw, I’m not a serial killer”. Well, he is saying that he is NOT a serial killer, so it’s okay, but of course it makes the girl all the sudden shift her focus to process a scary question -whether or not he is a serial killer.
The analogy is not perfect, 1. cause it would not probably be socially acceptable to say the serial killer thing, while the consent thing is, and 2. the serial killer scenario is the person saying smth bad about themselves, while the consent scenario is the person accusing the other of a bad thing. The analogy is for the fact that in both cases you are shocking the person by making them suddenly consider a horrible thing, even tho you are negating it in the same phrase.
I’m also assuming that u are saying more or less abruptly (eg before or during foreplay u are saying smth like “btw, I do give consent”, that pulls torwards the negative interpretation), versus smth more smooth like “I would love to have sex with u!” that maps obviously to the positive interpretation.
Maybe u should think of ways to be more smooth about it, cause unlike u I think the vast majority of guys are well intended