I got beaten up growing up in Puerto Rico..?

I got beat up often growing up in San Juan Puerto Rico and consequentially i learned to fight better than average. A couple of years ago i got into a fight and choked the fellow out and my friend (male) freaked out and never talked to me again. If im going out with a female, and some fellow grabs her behind, im worried if i react she might freak out and do the same-never have anything to do with me again. Ive lived in thailand for most of the time since the afore mentioned incident and this has never happend there but im going to school now in the usa so ill be here while.. What if this happens should i let her handle the situation or trounce the fool myself and hope for the best?