I'm not going to mow the lawn for my roommate's girlfriend
Why don't you want to mow the lawn?
I only want to please my woman. She is not my woman.
It is understandable why you are upset, your friend shouldn't ask you for favors so that he looks good to his girlfriend. You should talk to him about that so that he won't do it again.
He's her boyfriend, so shouldn't he be the one to mow her lawn?
I agree, he should not take advantage of you and you should let him know this in a firm but polite way.