And it makes sense in-universe because her way of showing interest makes him simultaneously both a} genuinely horny and b) genuinely uncomfortable (and there's a massive feedback loop between the two). He wants to let down his guard and is terrified of doing just that.
outing myself as a combustible normie, but man. this brought back some some really nice feelings. girl doing something nice for you unexpectedly. even better than sex.
Quick, author! Introduce a male character that is naturally close to the girl so the MC feel threatened whilst confused in his own feelings! Quick, while the readers are in awe with their relationship!!!