Free spirit Justin Theroux has started to see how high-maintenance Jennifer Aniston can be. Case in point: her obsession with being thin. The actress, 42, recently quit smoking in hopes of getting pregnant, but it caused her to gain 10 pounds, so now she “complains nonstop about how fat she is,” a source tells In Touch. “Justin really can’t believe how vain Jennifer is. He will want to go to a restaurant and Jen will say that the food’s too fattening or that she can’t stay up late because she’s getting up at 6 a.m. to go to a workout.”

Despite her exercise compulsion, Jen isn’t breaking a sweat in the bedroom. Desperate to conceive, the actress has instituted a sex ban, insisting to Justin, 40, that they should only have sex during her most fertile days each month. As a result, “Justin has been saying that their love life is more like a science experiment and he’s really fed up with it,” says the source. “It’s hard to deal with her neurotic side.”

After all the drama, Jen may in fact be flying solo soon — at least for the holidays. Jen made plans with Courteney Cox for Christmas Eve, but Justin “flat out said no,” says an insider. Although a rep insists they’re fine, the friend says Justin had finally had enough. “He sort of rebelled and said he wasn’t sure about going,” the insider says. “He’s starting to feel like Jen is scheduling all of his time up so that he won’t be able to see his friends,” says another pal.

Jen also thinks it’s “unhealthy” that he still talks to his ex, Heidi Bivens, says her friend. Indeed, if Jen doesn’t loosen up the reigns on their relationship — stat — the pal adds, “They may end up spending time apart.”