1. they look similar. then it's just a tribute.
2. they're twins
3. it's with a celebrity
4. it's a threesome
5. you spread peanut butter on your balls and your dog licks it off
6. you don't remember
7. you don't look them in the eye
8. you get away with it
9. she says it's ok
10. it's with royalty
11. you have two girls at the same time, so one invalidates the other.
12. you can please all of them
13. it was actually them but you were thinking of someone else
14. you're in a different area code, not to mention a different state.
15. you go home afterwards and fulfill your duty.
16. It's in a different ZIP code
17. she's not human.
18. you change the sheets afterwards.
19. you get paid
20. they're on top