2- Lack of intimacy
It is totally normal for your bedroom activities to slow down with age, or because of your work schedules, maybe you have kids and are utterly unable to have any alone time together, but to lose any sort of physical intimacy, to the point of not even being attracted to your partner that way is a really bad sign, and chances are your relationship is bound to fail miserably, one of the major aspects of relationships is physical intimacy, it is something shared between only you two in full exclusivity, otherwise you’re reduced to co-parenting roommates, it is very important to have a healthy level of physical intimacy for one to sustain a healthy and loving relationship.
One other aspect that is often neglected is emotional intimacy, although physical intimacy is important and takes all the light, being able to be vulnerable with your partner, and talk thoroughly and honestly about how you feel is also invaluable to any relationship succeeding, and in consequence, a lack thereof is a sign of an imminent breakup.