If it goes on particularly long it can become a “hey, my needs aren’t being met, can we talk this out” situation. Going a long period with your partner not wanting to have sex can fuck with your head even when there’s good reason and sexual incompatibility is a perfectly valid reason to walk away from an otherwise healthy relationship.

But also yes, “I’m not in the mood” is a perfectly valid reason until it hits the point of someone’s needs not being met, and it remains a perfectly valid reason not to have sex after, it’s just that I’ve got no judgment for someone walking away over it when it gets to that.

Having reasons just shifts the needs into a needs vs responsibilities conversation and opens the situation to more sympathy. “I haven’t felt like having sex with you this year” can be kinda devastating and can lead to questions like why and will this be normal. “I’ve been in a depressive episode and not had the mental energy to want sex for this past year” still sucks but it cuts out the fear that it’s something wrong with you and addresses that the situation is ideally temporary. By contrast “I’ve been questioning if I’m asexual because I don’t know if I ever really wanted sex, or just wanted to want it” serves as a very good reason, but one where it becomes clear that this is unlikely to be a temporary situation and allows the other partner the agency to decide if they want to stay in a relationship where sex is either off the table or a rare occurrence or if it’s time to look at the possibility of amicable separation.