Being unable to lie means relationships are about physical attraction?
I get the plot was probably an afterthought in writing this movie, but it should still make sense. I couldn't wrap my head around it. What did being honest have to do with having a meaningful relationship?
I get Anna was out of Mark's league and didn't want to date him because of that. Well, that happens in real life where people lie. People generally date people in the same basic "league" and what league you are in is determined by several different factors, like attractiveness and wealth. That doesn't mean people can't find love or a meaningful relationship. And it certainly doesn't take lying to find love or a meaningful relationship. Lying in real life isn't going to change what league you're in anyway. It only worked here because everyone only told the truth. It gave him an advantage. It didn't level the playing field.
But not all women in real life are looking for a mate who can provide good genes for their offspring. In fact, very few are. People don't often choose good looking partners because they want good looking kids. They choose good looking partners because they like their partners to look good. It's much more simplistic in real life. There's no need to invoke lying. If she was worried about kids she could have chosen Mark because he was smart (which she said repeatedly) and could give her smart children. Though this is operating under movie logic. In real life, it's well known that your child's genetics is very much a lottery. It doesn't matter much how good or bad your partners genes appear to be (phenotype). You have just as good of a shot having an attractive, gifted child with an ugly, ungifted partner as you do with an attractive, gifted partner. It would be a lie to pretend otherwise. Ironic, given how much of the film's premise depends on this idea.
People do choose mates because they think they'd make good parents. Usually these mates have nurturing qualities. People don't often select potential parents of their offspring based on potential genetic qualities... Well, they do; but like the movie pointed out, they use sperm donors.
If Anna was one of the few women in the world who wanted some genetically superior superbaby, then it's kind of a jerk move for Mark to manipulate her into something else. He should be able to respect her choice and move on. There's plenty of other women out there who wouldn't place so much emphasis on his looks. Many might prefer a smarter man. Though this is assuming this world works like ours. Given how it was set up, the movie makes it seem like Anna's position is the norm. In which case, the question is: WHY? There's nothing inherently necessary about lying to have a healthy, meaningful relationship built on love and friendship. For a movie that's stretched out from a single, simple joke they really needed to tie the relationship plotline (which drove the narrative) into the theme of lying/honesty better. As it stands, it's just a poor excuse to mock religion... and I'm an atheist, I shouldn't be pointing that out as a downside. But in this case, it just wasn't enough.