Physical Theory: Nameks firstly do have an apendage similar in size and shape to a penis (just no ballz), this is used to dispell water from their bodies, just as mammals urinate. We of course do not know of their apendage because pants always conceal them. Now that we have that aspect out of the way, onto making babies...

The way Namekians can clone themselves is by way of the meeting of two single cells, one is located near the neck, the other close to urinal opening. They begin to grow as the Namek wills them, the 2 cells forming as one inside the neck after meeting, and becomes an egg. A similar things happens if the cell down below purposely or accidently is implanted in a woman. The cell, desperate to form with something, locates the ova, entering it as a sperm would. The growing process would decrease as new DNA is formed - remember, it only contained HALF the information of the Namek, just as sperm contributes half the info of a man. Within 9 months you'd have a baby born as mammal would.

As for erections...heh...let us not forget how well Namekians can will and control their bodies. For an erection they'd have to will it themselves, however, I image it's a delcate area, so I'd say they'd feel a great deal of pleasurable sensations.

Does that all make any sense to you...? It's my view on things, and I thinks it's a good explanation on how Piccolo could inpregnate Bra. Now I'll get to the more romantic aspects, why they'd be attracted to eachother.

Emmotional Theory:

BRA= All big DB fans knows how much of a daddy's girl she is, and usually daddy's girls end up marrying a man most like their father. In the DB Universe, Piccolo comes closest to Vegeta, he shares a past of evil, is driven to be the best and are both have similar personality traits. If you ignore the green skin, Piccolo's not a too bad looking guy, because he's so tall and strong, she may find this attractive, especially the fun of the challenge to get someone like himself. He is a very dependable, honorable, capable and truthworthy person, whom Bra would know she could always count on, and though he may not be open about his feelings, when the moment's right he always shows true emmotion.

Piccolo= Piccolo has never even considered a relationship out of friendship and the fatherly love he has for Gohan, romantic love would add another dimension and help him to discover new aspects of himself, even increase his powers. Bra, with her whimsical and innocent in intent would be able to open his heart to romantic love. Though he is much older than she, in retrospect he's quite young considering how long he'll live. The upside is she'll also live a long life thanks to her Saiyan blood, but it won't come close to the years he'll live. Piccolo is drawn to be around the innocent, it is shown by the people he most associates with. He is most sociable with the Son family, especially Gohan and Goku, and lives in peace with Dende and Mr Popo. So, it is fair to say he'd also find himself drawn to Bra who is innocent, yet able to match his temper with her own, inherited from her mother. A more humorous point to why in the back of his mind he may enjoy being with Bra is the simple fact it would piss of Vegeta beyond belief. Seriously though, I can see he and Bra being very compatible and having a loving relationship.