I’m a man and I pee sitting down because:

it’s more comfortable

Agreed, if there’s a toilet (as opposed to a urinal) i will generally sit down.

It’s solely a matter of comfort and/or convenience:

• No. 2: Toilet (obviously, I hope).
• No. 1 on the road or at the pub: Urinal or standing elsewhere.
• No. 1 otherwise: sit down and relax.