There's a serious problem with the liberal moral framework once we apply it to relationships. If the individual is sacrosanct, then personal pleasure and self-fulfillment take priority. After all, we were constantly told when we were young that we're super special and capable of anything (a lie). This is fundamentally unworkable in situations involving more than one person. Either all parties have to set aside their self-interest for the purpose of community, or everyone remains self-interested and relationships become "alliances of convenience."
Excepting abuse or distance, the vast majority of romantic relationships that I see fail do so because of the lack of honesty and deception made necessary by self-interest. Either one party wants to "see" other people, or they cheated on what was presumably a monogamous relationship.
If you want to "see" other people, you want to fuck. Don't lie about it, because the person you've just dumped has no good indicators for how to change their behavior. If someone is an undateable schlub, telling them that it's not THEM it's YOU reinforces the delusion that we're just fine the way we are. We are not. Not that person and certainly not you. You certainly can't act under false pretenses, either. If you're not being honest with someone because you're afraid it will terminate the relationship, then YOU are the scumbag for practicing deception in order to keep getting laid.
We all have a problem hooking up, some of us moreso than others. But the youth of affluent societies, where we can afford to live within a liberal framework, have a particular problem just being with people for lengthy periods of time. We're getting bored with each other, and I don't see how this is acceptable.
None of this is to say that we should return to some reactionary moral paradigm. Yet we have to be honest about what's going wrong with our lives and "loved" ones. We have to be open and honest in our interactions with others, and for hormonally charged youth it basically means that polyamory is the future. Why pretend that we really care about being monogamous? Monogamy should be a standard held only for people that synch perfectly. Giving your life wholly to one individual is the greatest expression of love I can possibly think of.
Monogamy should therefore be the ideal, not the standard. Be for real.