Your fears should be easily calmed because mother nature took care of your question. If your penis is erect, as it would be during intercourse, and you are about to have an ejaculation, you cannot urinate. So don’t worry about it and just allow yourself to have an orgasm. As to your stretch marks, remember none of us is perfect. If a woman likes you enough to have sex with you, she’s not going to worry that much about some stretch marks on your stomach. So relax and stop worrying about these things and enjoy yourself. Just make sure you practice safer sex, since it seems you are not in a long term relationship with these women.