Yeah, sorry, I have to heartily disagree with the decries for romance adversely affecting a story (ANY story) and somehow making it less serious. The desire to procreate and continue the species/bloodline is one of the three main driving forces of survival instinct, and in fact has been claimed to be the most important (the other two being shelter/protection and food/sleep/necessities of living), as the other two are generally sought out in order to more easily fulfill the first. It is completely unnatural for this instinct to be ignored or downplayed (as is the case in lustless MCs), and equally bad to tease it without fulfilling it ("cockblocking" either from other female characters or by virtue of the MC's own uselessness or personality). This desire has evolved with human cognition and ego where we both not only desire someone of the opposite gender (generally anyways) because of their appearance and visible traits that make them seem like they would be able to reproduce easily (reptilian brain instincts), but with the additional consideration of "romance" added into general attraction (mammalian brain emotions and cognitive thought).
Additionally, I posit this. Fighting for honor or to protect the general population (aka, no specific people) tends to ring very hollow. It's much more personal and powerful of a motivation to fight to protect your family, friends, and/or lover(s) (especially a lover), and developing a romantic relationship DURING the story gives a character something to look forward to when the fighting is over, which gives them even MORE motivation to fight and fight smart. Even a dark and serious story has to have some light of hope to it, otherwise it's just a pointlessly masochistic circlejerk tragedy.