The specifics of how Vegeta and Bulma first slept together are never explicitly shown in Dragon Ball. However, it is heavily implied that it began as a one-time, spur-of-the-moment occurrence.
Implied Circumstances
- Stress Relief: Given Vegeta's personality and the high-stress environment they were living in, a moment of passion spurred by the constant threat of enemies seems plausible.
- Unexpected Connection: Neither Vegeta nor Bulma may have initially intended for it to become a relationship. It likely started as something casual.
- No Romantic Overtures: There were no prior romantic overtures shown, which suggests the initial encounter was driven more by immediate needs and perhaps a developing, unspoken attraction.
Lack of Explicit Details
The Dragon Ball series generally avoids depicting intimate details. The focus remains on action, adventure, and character development through combat. Therefore, the circumstances of their initial encounter are left to implication and interpretation.
Consequence of the Act
Regardless of the circumstances, the outcome of their encounter was the birth of Trunks, which solidified the beginning of a complicated, yet ultimately loving, relationship between Vegeta and Bulma.
