grossed out by sex

i’m just going to get right in to it. i’ve had boyfriends in the past, i’ve had sex a lot of times. i’ve hooked up once and i’ve always been weird around semen. no matter what i’m just grossed out by it. i had a hook up a few days ago and my weirdness around it ruined the whole thing for me and him. he was about to finish on my stomach and i said “wait” and covered my stomach but obviously he finished anyway and it got all over my hands. i started gagging and freaking out and went to wash my hands immediately and asked if he got any on my sheets. i felt so bad and i was angry at myself for ruining it for both of us. he said it was okay but i still don’t know what my problem is. any advice helps