Fat heteros

Marriage and co-habiting cause obesity, if you are a woman, but makes you healthier if you are a man.

Men found themselves eating lighter, healthier meals and more fruit and vegetables, while women moved towards the creamier, heavier dishes like curries and rich pasta meals favoured by their partners.

Men, of course, need more calories than women, so eating the same thing together is causing more weight gain in women. But marriage also discourages exercise by making less time available for it.
Stereotypically, gay guys go to the gym together and lesbians organise healthy vegan gluten-free pot luck dinners for social occasions, which leads us to conclude that old-fashioned monogamy is bad for your health. Not that there aren't any obese gay men or lesbians, I wouldn't want to take the stereotypes too far!