Well, to my memory, Future Trunks described it as a moment of passion thing. In the mainstream timeline? The sexual tension started building up while Vegeta was training for the Androids saga, kinda a Florence Nightengale syndrome thing on her end. On his end the one of the major things that attracted him to her were likely the simple fact that she actually seemed to care about him to that extent, and even showed signs of interest in him from time to time. Considering that he spent most of his life under Frieza's thumb, and most of the other earthlings still regarded him with a mixture of fear, distrust, and anger, that was likely a very novel experience for him. Bulma also rather uniquely had a force of personality that kept her from being cowed by Vegeta's own abrasiveness and even allowed her to turn the tables on him a few times. In other words, she unknowingly presented herself as an equal. Mind you, this is largely inference.
