It is enough of an imbalance to cause fierce competition amongst the men. Assuming a nuclear family and sensible family planning, it is highly likely that a bunch of men will not get to have families.
The men working hard to make money are in a fairly high-stakes game to break into the gene pool, biologically speaking. It isn't surprising to me that the men take working a lot more seriously. Every generation nature is roughly planning to cull ~1 man in 20 before life even starts. Individually it might not get acknowledged but that is a mighty biasing force over even 2 generations of cultural development. Being the lowest status man in the room has an implicit evolutionary threat in the way that being the lowest status woman doesn't.
What perceptible "evolutionary threat" could there possibly be in a 105:100 imbalance, which is so low as to be pretty much unknown to everyone who doesn't follow demographic data?
Rank them by the security of their relationship with their girlfriend or wife. The first few without a partner (the marginal ones) would in all likelihood have partner if they were an equivalent girl. There is a lot of incentive to behave competitively.
[0]: https://science.sciencemag.org/content/362/6412/eaas9899