Obviously I can't speak intelligently on what it's like to have breast large enough to be any kind of issue when riding any more so than a woman can speak for what it's like for a guy, but if your boyfriend finds it humorous he'll be just as amused if you go jogging. Just tell him the options are having breast large enough to bounce or having them small enough to not bounce. While sizes can vary it's not like women have much say in what nature provides.
As for people seeming to think that males suffer a high rate of potential injury to our "privates" in 4.5 decades I've had exactly one occasion as the result of a one off incident involving a horn that caused an unfortunate impact. Didn't prevent me from riding the rest of the day though. Aside from that I've never had a problem. Never even think about it. So far I don't personally know any guys who has had a problem, but I can't claim to know every male what rides
. I'd have to seriously wonder how a guy who has problems is riding. Can they sit on a chair without injury? Are they able to ride a bike without injury? I personally find sitting in a saddle more comfortable than anything except perhaps my favorite lay-z-boy and possibly easier to spend the entire day in.