I hate to be the one to crash the "EWWWWW!" party, but there's a very good explanation for why this happened - and it's not particularly rare, either.
It's called genetic sexual attraction. It's a common occurrence when long-lost blood relatives are reunited. They frequently describe the attraction as "an overwhelming rush of emotions combined with the irresistible sense of falling in love". This attraction is likely based on mirroring (attraction to our own characteristics in others) and a confused desire for closeness given the natural family relationship was disrupted by separation.
So yeah, it's a hardwired reaction, which is why it's usually a very bad idea for a mother and a son to reunite, unsupervised, when both are entering their respective sexual primes. Their emotions, combined with their hormones, easily overwhelm any sense of wrongdoing, especially in the absence of persistent social markers - such as the boy's adoptive family.
Anyways, here's a good way to think about just how powerful this attraction is for these people. Think about a guy or gal you're extremely attracted to for no good reason. He/she might be cute to the rest of the world, but you think he/she is gorgeous beyond compare. You're drawn to him/her in a way you cannot fully describe - almost like you were meant to be together. When you're around each other, everything just "fits". Sounds a lot like true love, doesn't it?
Well all of that works perfectly within the context of a mother reuniting with the young man she gave up for adoption some ten years ago. He's unrecognizable as her "child" now, and she didn't see the progression. All she sees is a young man who mirrors a lot of her own characteristics, and for whom she feels an undeniable urge for closeness. When two adults (grant me that he's sexually mature) have that sort of connection, the expression of said "closeness" typically involves the horizontal mamba.
Look it up. Plenty of sob stories about long-lost siblings or parent/child pairs where one or both develop obsessive feelings after reuniting for the first time.
Sidebar: it probably doesn't help when she's a certified hottie.
