Although it's not fair to only pick on Ashton Kutcher since hundreds of celebs have said the same thing, here goes: In a recent interview, the actor was asked if jealousy ever arose on either his or Demi Moore's part over love scenes they have starred in.
It's interesting that people look at sex in that way. If I had a scene in a movie where I had to go shoot someone with a gun, Demi wouldn't think that I was a murderer. So if I have a scene in a movie where I'm having sex with somebody else, Demi doesn't think that I'm in another relationship. You go to work and you play make-believe, and you come home and you live real life. That's part of our jobs.
Sex with a beautiful co-star is like shooting a gun? Really?
Mayo Method isn't that stupid. She was happily (presumably) married to 45-year old Humphrey Bogart when her husband co-starred with 19-year old Lauren Bacall in To Have & Have Not, in 1944. The next year they weren't married anymore and Bogart wed Bacall.
More recently, Brad Pitt, married at the time to one of the world's most beautiful women, Jennifer Aniston, fell in love with his co-star Angelina Jolie after one of those cold, mechanical, love scenes that Kutcher likens to gunplay.
I could go on -- Hollywood history is replete with such examples, but Kutcher is ignorant of all of that and lives in a reality-free zone wherein human beings are unemotional creatures who can perform sex scenes without any emotional attachment.