What would you do if you saw a man in a public restroom?

Well if he/she/it wants to be a woman, offer him/her/it a pair of scissors and an invitation to stay out of the women's room until he/she/it is in fact a she.  

Let them go into a cubicle and use it for its intended purpose.

If they cause a nuisance of any sort, report it to authorities.

Dayton, women’s toilets don’t have urinals.

Get on with what we were there for, and walk out.   What else?   Unless he/she attacks you/your daughter.

All the lavatories in in separate lockable compartments so it should not be an issue. For the most part how would you even know what their gender was supposed to be?

I'm a guy, so I would try to leave a larger gap at the urinal, ideally 3 empty spaces.

We would walk out.

If they're exposing themselves, probably leave and call the police. Otherwise they are in a stall. We've all been in restrooms with trans people and don't even notice it. If you find yourself looking at people's genitalia when you're in restrooms, you should see a therapist before you get in trouble. 

Tell my daughter to enjoy it.