Well, according to Brennan/Piler/Braga and later Braga's histories with love stories in Star Trek franchise for TV from TNG to Entreprise, anything could happen: from a successful romance which ends with a beautiful wedding and a child to a love story which ends dramatically to a classical break up to try again with someone else . Every scenarios are possible -> a lot of people hoped/expected Janeway/Chakotay, at worst, in the finale series and however, they ended with Chakotay/Seven! So...
And the sort of potential I saw for Tom and B'Elanna, outside of getting married? A deep and tender bond between 2 people who have learnt to know and understand each other, that better than anyone, including Harry and Chakotay, after to have first been friends then lovers then friends again.
For centuries, to get married and to be parents were top priorities. Now, marriage and parenthood aren't an end in itself, contrarly to get a life (to be professionally successful, to thrive a social life, to live alone or accompanied). Is this evolution regrettable? Maybe but that's how it is. C'est la vie! :-D
You know, life holds many surprises... But in admitting that Tom and B'Elena were made for each other, I would have prefered to see them together for several months then due to their different personnalities and lack of compromises between the workaloclic B'Elena and the boyish Tom (or because what's happened in Nothing Human (5x12) and later, in Lineag (7x12), where trust & fear created a deep discomfort between them), broke up and after 1 or 2 unfortunate experiences, to come back together to try again in a more mature relationship and a few time later, finally ready to take the plunge. In short, a relationship which which advances step by step ( with its advances and retreats). it would have been more realistic.