Once you have completed Uma's championship boxing quest you can stop by the boxing gym at 1PM on Monday, Wednesday and Friday for a sexy good time with one of the fighters. Monica is my favorite and appears to train every Monday. The others have their own day and you can have Uma any day I think (just has to be before or after the other fighter's gym time on Mon, Wed, Fri)... I just remember to show up for my Monica Mondays and do the personal trainer bit the rest of the week. Hope that helps you friend.
Adventure ever on, Phat
PS: Be on time! That means wherever you are at noon on a day you want to encounter one of the fighters, it is time to go to the gym! By leaving any other location at noon, you will arrive precisely at 1 PM for your "boxing training."
Also, you cannot have the boxing date if you have trained in any other way on that day. You only get one training session per day. So, if you go play personal trainer in the AM, no boxing fun for you at 1PM!