"Real life" doesn't even remotely resemble the ancestral environment. In the modern world, a big family is evidence about your cultural background, especially the relationship between your cultural background and contraception, and that might be a turn-off for some. This is the same kind of phenomenon that makes having extra fat evidence, in the ancestral environment, that you were good at acquiring food and other resources, but in the modern world it's evidence that you're poor or lack access to good food or lack self-control or whatever.

Why does evo. psych imply this result? The fact that you can spawn healthy children may make you more attractive to an additional potential mate, but the potential mate must also consider whether you can provide resources to support additional children. [...] This seems like the opposite of what evo. psych would predict. Your relatives might be willing to provide you more support if you have more kids, but why would strangers be more willing to support you based on your reproductive fitness? As for relatives of additional potential spouses, the considerations of I discussed above still apply.