Is it muttar to attend professional baseball, basketball etc…games?
Rav Moshe Feinstein in a famous responsa prohibited this practice based on Bittul Torah and Moshav Leitzim, basically wasting one’s time and engaging in frivolity. Some Poskim hold that when there is no problem of Bitul Torah as the person needs some recreation in any event, and the venue is clean of improper images, etc. it would be permitted. It is possible Igros Moshe would agree to this as well. Today at some sporting events there are problems of immodesty either from the general crowd or from secondary forms of entertainment. This is a different problem, beyond the above, which would forbid attending in it’s own right, but is not applicable to every event.
Igros Moshe Y:D 4:11, see the psak of Rav Sheinberg quoted here.