Romantic relationships are not an exact science. However I came across this rather apposite advice called the '3 C's' that need to be there for a relationship to potentially work:
(1) Compatibility: if you and your partner's sensibilities clash too much this could spell trouble in the medium to long term and end up in misunderstandings, arguments and general interpersonal neurosis.
“Αί γὰρ ἀνομοιότητες τῶν ἠθῶν ἥκιστα φιλικόν.” - Aristotle, Economics, IV:2
("Incompatibilities of habits/characters are far from favourable to friendliness/affection.")(2) Commitment: cheating on a partner is a form of betrayal unless they've given you the green light. Even then it could lead to hurt for the party you're cheating with. And they do say two's company, three's a crowd for a reason.
(3) Companionship: ideally a successful relationship results in both partners being mutually respecting life companions, happy or at least not unhappy in each other's company, weathering the good times and the bad together as a unit and providing mutual support, morally, emotionally and financially.
Great sex is of course a bonus, and not a negligible one at that, but I do not see it as an essential component to a great or even moderately successful relationship, if one of these C's is missing. If having kids is an utmost priority for one partner and not for the other this could spell trouble but I would put that problem in the first category 'Compatibility'. Some people are of the opinion that no sex means no relationship but I am not of that persuasion.
To take this post a little deeper I would say that a relationship based in truth, a true relationship so to speak, is the ideal.
"Let me not to the marriage of true minds admit impediment."['True' in this case potentially meaning both enlightened and faithful.]
For this to occur both partners must know themselves and be enlightened.
Erotic love (έρως), even love of one's neighbour (φιλία) are not equal to the love of truth (ἀγάπη) which I view as the ideal basis for a successful relationship.