It is just the plumbing interacting with the basic primitive wiring.
Women release oxytocin during orgasm, which does have them bond to their partners.
The same hormone is released during childbirth.
All of these hormones have receptor sites that overlap with the dopamine 'reward' centers in the brain.
Messy biochemistry really, but it has kept us propogating the species for hundreds of thousands of years.
The goods news, if you survive this set of addictive hormones after they wear off (cause they do and most people think it is just 'falling out of love') a whole new set kicks off, not as intense, for further pair bonding.