The actress, 38, opened up about the topic on a podcast alongside husband Cash Warren, 40.
She cited two Hollywood actors she has worked with to illustrate the point, reports dailymail.co.uk
"I think Pierce Brosnan and Bruce Willis are very attractive gentlemen... but it's like, weird," said Alba, who worked alongside Brosnan in 2014's "Some Kind of Beautiful", and Willis in 2005's "Sin City".
The mother-of-three differentiated between a romantic encounter in one's personal life, and one on a film set in which "nothing about it is alone in a room", and can go on for 15-16 hours.
"When you're choosing to want to be with someone in that way... it's very intimate," Alba said.
"You're breathing on someone, it's all there, you're smelling the skin ... you wanna choose who you're doing that with, and you gotta really like, pretend."
The Honest Company head said that "sometimes it's days and days of the same thing, and you have to recreate that moment every time, recreate the peak of your intimacy from this angle and that angle and this angle and that angle ... and by the way, everyone's right here".
The "Dear Eleanor" actress said she has to take tequila shots prior to such encounters.
"I'm gonna say I'm unprofessional, 100 per cent: because I feel like really good entertainment people don't have to do that," she said.
( With inputs from IANS )