Electroencephalograms taken of men and women during sexual intercourse show that orgasm resembles "a kind of pleasurable epileptic attack." One loses control over one's muscles. Vegeta has been known to destroy entire planets. What would he do to the woman in his arms during what amounts to an epileptic fit?
Consider the driving urge between a man and a woman, the monomaniacal urge to achieve greater and greater penetration. Remember also that we are dealing with saiyan muscles. Vegeta would literally crush Bulma's body in his arms, while simultaneously ripping her open from crotch to sternum, gutting her like a trout.
Lastly, he'd blow off the top of her head. Ejaculation of semen is entirely involuntary in the human male, and in all other forms of terrestrial life. It would be unreasonable to assume otherwise for a saiyan. But with saiyan muscles behind it, Vegeta's semen would emerge with the muzzle velocity of a machine gun bullet. (*One can imagine that Prince Vegeta's Palace on Planet Vegeta was riddled with holes during Vegeta's puberty.) In view of the foregoing, normal sex is impossible between Bulma and Vegeta.
Let's cheat a little and say this is all somehow possible up to insemination. A ripened but unfertilized egg leaves Bulma's ovary, begins its voyage down her Fallopian tube. Some time later, tens of millions of sperm, released from a test tube, begin their own voyage up Bulma's Fallopian tube. One sperm arrives before the others. It penetrates the egg, forms a lump on it's surface, the cell wall now thickens to prevent other sperm From entering. Within the now-fertilized egg, changes take place...
And ten million saiyan sperm arrive slightly late.
Were they human sperm, they would be out of luck. But these tiny blind things are the spawn of saiyans, warriors capable of strength beyond anything else in the known univers. A thickened cell wall won't stop them. They will *all* enter the egg, obliterating it entirely in an orgy of microscopic gang rape. So much for cheating up to insemination.
Within her body there are still tens of millions of frustrated saiyan sperm. The single egg is now too diffuse to be a target. The sperm scatter. They scatter without regard to what is in their path. They leave curved channels, microscopically small. Presently all will have found their way to the open air. That leaves Bulma with several million microscopic perforations all leading deep into her abdomen. Most of the channels will intersect one or more loops of intestine. Peritonitis is inevitable. Bulma becomes desperately ill. Meanwhile, tens of millions of sperm swarm in the air over Capsule Corp.
Okay, this is getting boring. But ya, you get the picture. Everything from kissing to making Trunks and Bula = No no.