Stimulating the urethra wouldn't do anything to my knowledge, bu then again, anal sex is enjoyable, so maybe you've got something there.
If you feel like an orgasm is coming, then... It is! Now the thing is, does the missionary positon you're in stimulate your clitoris, or does it stimulate inside the vagina? If it's inside, then you're having a "g-spot" orgasm. If its outside, you're having clitoral orgasms.