None of the above at this time.

You're not done processing this thing. I'm guessing you think about these other girls when you're not masturbating as well but when you're sexual and thinking about your boyfriend for some reason you're not the one with him. If it's distressing to you that this is the fantasy you have, then this isn't something you're attracted to. It would be very unsual that the only place your mind would deal with anger or frustration would be masturbation. There are so many more likely outlets for stress about your relationship. Maybe being cheated on has become sexualized because of your inexperience at the time, certainly traumatic experiences can make you look at a circumstance differently but generally when something becomes a kink for you it's not a maybe thing, you feel it like turning on a lightswitch, that certain.

Keep talking with your guy about this. Confide in him that his affairs are turning up in your masturbation headspace. Ask for a little more reassurance and security in the relationship and really work at putting the affair behind you both. If this drags on or if your trust seems to deteriorate you might talk to an analyst or counseler about why this is sticking with you and how you can processes it better.

the mind can be a funny thing. Those thoughts are pinned in your head! This could be one way that your dealing with it.
why think of that rather than killing the women, i dont know.
i wouldnt put too much thought on it, try to change your thoughts to something else the next time.