So Aidra was engaged, and had sex (and got pregnant) before the wedding actually took place (and with a total scumbag too - why!?) ... then comes back and Bard helps raise her offspring for 30 years without a relationship ever developing between the two. Why not? He's the most famous Knight in the world, surely that would be suitable enough match for Aidra, who had at that point given birth to a bastard child.
They could have had 30 years of sex and half a dozen children together, so why not? Erg. Why do people make such bad decisions?