I'm not sure if I agree that being a parent is a bonus.
I had a friend, male who was happily married. He cofounded a company with another right before he had a kid. Because his wife was the main breadwinner, he often times had to work-from-home to care of the baby. The needs of his company barely overlapped with the need of the child, and eventually it got to the point, where he ended up leaving the company to focus on the child.
The company he cofounded has since died.
You're right that some VC's will unfairly connect woman with motherhood, i.e. "Ah you're a female founder so you're going to have a kid and then the company will die!"
That is sexism, no doubt. But parenthood, regardless of gender, will impact that founder's level of commitment to the company, especially if the founder is the main caretaker of the child.
If this were an established company, yes: both parents can work and raise a kid well. But a startup asks for more than a 9-5 commitment: it's like another child that requires constant attention and love. Its specific needs differ greatly from human children[1] but those needs are just as pressing and as demanding, if not more so. Anything less portends an early end.
[1] Baby food v.s. VC funding.