Emotions and Feelings
Bio-Androids seem to take on a will of their own. From what I've seen, they don't follow a master's order directly, but rather live according to their own beliefs. Questioning themselves is a common feature, most don't know if they are more machine than man. But they apparrently have feelings, you can see them laugh, cry, despair, and hope among others.
Living Material in Addition to Mechanical Parts
Some sort of living structure has been incorporated to form the Android. In Wild ARMs, Rudy was made of a special type of living metal. In Wild ARMs 3, Jet had some of the genetic makeup of Filgaia (their planet) inside of him. And Melphina from Outlaw Star seemed to have real human DNA. The living materials give them a look that makes it hard to identify them as Androids, if you didn't cut open a piece of their body. This includes skin that looks and feels just like real flesh.
Love, Marriage, and Children
Oh, am I ever gonna have a field day with this one. Now, a Robotic Android is incapable of feeling real love (for the most part, but there are exceptions). But Bio-Androids are constantly falling in love in these shows and games, if not there then in the Fanfics. Due to their free thinking and living genetic makeup, they tend to find an attraction to another character (usually, the Bio-Android is one of the two main characters, and he/she falls in love with the other main character). Rudy can be plausibly hooked up with either Princess Cecilia the sorceress, or Calamity Jane the gunslinger. Jet looks just too kawaii with Virginia (he does save her on several occassions). And Melphina learns of true emotions from Gene. Now, about children...I do think they can reproduce. They are practically human as it is. So they must be able to do something of the like. Heck, if the technology that created them made them this advanced, it has to be true. Besides, it's so cute to draw what their kids would look like.
Lifespans
Yes, I believe these creatures actually do have a human's lifespan. Witness how Rudy from Wild ARMs was seen as a young child in a flashback, as well as by Cecilia when she saved him from his dreams (shown more prominently in Alter Code F since the graphic engine was better). But he is a teenager in the game. If he had always been a teen, I think he'd be questioning why he had been fifteen for so many years. Or if he was a child when he met Jack and Cecilia, I think he'd wonder why he wasn't growing up. Which would lead him to question himself much sooner, and could change the outcome of the game. Ok, so Jet and Melphina were created with more grown up bodies, but we never saw the Jet of a few years ago (after his birth), and you see Melphina just as she is being born. So we can't go by that unless we watched them for a few years to see if they aged.
Only one antithesis to this theory is shown...Vash from Trigun. He's been alive for about 120 years. However, he did age from a child. So he partially supports this argument. And countering that, Lita from Atelier Iris is definately living a human's lifespan; her own creator, who is like a mother to her, said so in a flashback and holographic message. But if I don't know, then I want to have them age with their peers.
That's my theory on Bio-Androids. If you don't believe it, that's your perogative. Heck, I just think too deeply into things.