if you leave Tracy during the gang bang, she will leave with Brian Reynolds (of van Wilder" fame) and he was mentioned before, when you first got to the club Tracy spotted him.
If you cum in the doctors mouth instead of joining Tracy's gangbang AND you have cheated with Lisa, you will ge the ending where Tracy is mad and wants to make a cuckold out of you.
If you stay with Tracy and don't cum until the end of the gangbang, you will get the happy swingers ending.
If you leave Tracy during the gangbang AND you don't make it clear to the guys involved that Tracy wants to be treated with respect, you will get the BAD gangbang ending.
If you never do the Vegas bukkake or the swingers club, AND you never cheat with Lisa, you will get the second dinner party with Derek and Lisa, and have the sex scene with Tracy after, with Derek spying.