Could being a guy and having a colonic make you gay?

No: I can't recommend it, though. The claims made by fringe practitioners at enema parlors ("colon cleansing") are false, and having this done to you is degrading and a waste of money. This will tend to get you involved with the larger world of unscientific practice. By the way, plenty of straight guys explore anal eroticism, and if you're through puberty your primary orientation is probably set.