you have to remember that they literally met at what was basically a matchmaking party w marriage in mind. they might not be a thing where you’re from but it’s more common in places like Japan where there’s a huge social pressure to get married, among other factors (like “herbivore” guys and a shrinking birth rate)
It would be sth like asexual... not exactly but kinda in practical ways. You can search for it as "hikikimori" on google too for further info. They are people who are not interested in having a real relationship or even having a casual partner. They have themselves to release their frustrations but they don't usually feel that kind of desires. The idea of calling then "herbivores" comes from that: they don't tend to involve themselves with other people to have sex (they don't have meat). If I'm not mistaken it's such a common thing lately that even the goverment is worried.
first time i've read a bl that actually expresses real life issues like adoption, same-sex marriage, surrogates, etc. it's very refreshing!