Does my boyfriend want to have sex with me or?

Yesterday night when my boyfriend and I were making out my knee accidentally hit his crotch and I could clearly feel and erection. It was hot and heavy and a lot of saliva was involved but I've never had an erection when we do this. I don't know if he just wants to have sex with me or he's just enjoying himself, but sometimes he slips his hands into my underwear an rubs my bum very sexually. I don't think I'm ready for sex and I don't think he is either cos we've only been dating for 3 months. Is it even normal to have an erection when you don't expect anything further?