As with any fandom, in particular those involved with ambiguous characters, the subject of Marron's potential romantic interest(s) is a heated one.
It is widely believed that Marron became friends with Goten, Bra and Trunks. This is, of course, negotiable since they were never seen together in their younger years, at least not in canon...but then again, Trunks and Goten were slightly preoccupied with saving the planet. In Dragonball GT, Marron lives with her parents at the Kame House, significantly far from the other characters. There is just one official image from DBGT depicting Marron and Bra standing together at a party, indicating that they might have been friends.
So who, if anyone, did Marron fall for during her significant amount of offscreen time? Trunks is probably the most widely-believed coupling, because of the closeness of their ages, and probably also the fact that Trunks is bishie, and it was convenient. (He can't date Bra, after all, and though Trunks and Pan has a heavy fanbase as well, there's something like 12 years' difference in their ages...) So Trunks is a distinct possibility, though I doubt that would have went over well with Vegeta. Trunks is a bit flippant in his younger years, and extremely busy with Capsule Corp as an adult, so whether he ever had time to date anyone will probably never be known.
Marron x Goten is also a popular pairing. Though he and Marron don't have that pastel aesthetic look together, they are fairly close in age, he is the son of Son Goku, and she is the daughter of Kuririn. It seems likely that both parents would support such a relationship. Goten is a bit of a ladies man both at the end of DBZ and throughout DBGT, and he may or may not have dated Marron.
The least-seen but still-there coupling found in the Next Gen fandom is Marron and Ubuu. This one baffles me, frankly, but anything's possible, I suppose. Since Ubuu's appearances in both series are minimal, it's very hard to say. It seems more likely that this pairing was born from the fandom's desire to have everything wrapped up in neat little packages (Trunks + Pan, Bra + Goten, Marron + Ubuu, though these couplings are frequently interchanged) rather than actual romantic interest. (If someone who supports this pairing would like to explain it to me, feel free to drop me an e-mail.)
It should go without saying that none of the Next Gen boys were dating her during canon Dragonball GT. Most of the fanon either ignores that series altogether, rewrites it, or proceeds to the end of it before spinning their tales. If Marron dated any of them previous to GT, obviously the relationship didn't last. And it seems unlikely that a relationship would be kept secret, despite her being fully human. In fact, if canon is any implication, at age 23 Marron is still living a very quiet life with Kuririn and Juuhachigou on the Island. She fears heights and dislikes flying, and was seemingly never trained as a fighter, despite her parents' professions. It's not only possible, but likely that she never marries or does so very late in life. Perhaps to Trunks, perhaps to Goten, or maybe to some regular human, much like herself.