My boyfriend punched me in the butt and says it was a joke... am I being too sensitive?

Wrenrose
I'm not sure if I'm being too sensitive here or if this should worry me. I'd really appreciate your help!
I've been with my boyfriend just 6months and we're happy in general. He's about 10yrs older than me and a LOT bigger than me (I'm little - about 8st and he weighs 13.5st). He is a joker - always kidding around and taking the piss and mostly mixes with other guys (lots of bikes andhe works as a welder so all quite macho). He'd been single for a while before getting with me.
Last night I was visiting him just lying on his bed - he came in ( in a good mood) and looked at my ass and said "shall I punch that" - I honestly thought he was joking so laughed and said no don't do that. Then we started chatting about something else and I quite gently disagreed with him and he suddenly punched me in the butt - it honestly did hurt and I have a purple bruise coming now. I told him 'ow that really hurt' and he just tried to laugh it off - he kept telling me it didn't hurt. I sort of stopped him and made him look me in the eyes and said 'you should apologise - that hurt' and he did but it took that for him to say sorry. Then the evening carried on exactly as if it never happened.
I've not really slept thinking about this. I am quite a feminine woman - I didn't grow up with brothers and I'm not used to rough and tumble and I like feeling like a guy is gentle with me not gonna hurt me. I don't want to chuck this early relationship away if my man was just misguided or I need a better sense of humour... but I don't want to gloss over a warning sign either.
I'd really really appreciate honest advice! Thank you!
Updates
+1 y
Not sure if this makes any difference to anyones opinion but this wasn't a sexy thing... yes we were in the bedroom but his kids and mum were in the house and we were all about to eat together... we were just chatting about where to walk the dog. .. so not rough sex at all. Just butt punching me in conversation :-D
My boyfriend punched me in the butt and says it was a joke... am I being too sensitive?
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