Hillary Clinton pressed the flesh of a guy who made a living flaunting his ... a bisexual porn star.

Clinton was in Texas recently for a fundraising breakfast when she mugged with Gavin Waters ... a retired porn star famous for his roles in "Splittin' Wood" and "Drill My Hole 2."

Waters dropped $1,000 to attend in large part because of her stand on sexual equality.

Gavin -- who also did straight porn -- has been retired from the biz for a few years and now works as a bartender at one of Dallas' biggest gay bars, The Tin Room.

It's no big deal ... just a fun photo. Porn pics are cool ... just ask Bill.