"I have a daughter" is a reason some men give for their feminism or sympathy with feminism.
It's one reason among many for becoming a feminist ally, but men should be careful of what is sometimes implied with it:
- that all fathers of daughters are feminists: obviously false, but sometimes fathers speak as if feminism naturally follows from having a daughter
- that it's understandable that men without daughters aren't feminists: just because your personal feminist beliefs arose from considering your daughter's life doesn't mean that all men do or should require such an impetus
- that the point of feminism is protecting a girl or woman you care about: feminism extends to the rights of all women regardless of your personal feelings for them, and protecting women isn't the point
- the problems of a young girl are different to those of adult women: and even if your daughter is an adult, she is not all women. You will need to base your feminism on more information than her described experiences or your imagined version of her future life.
- your daughter likely shares several privileges with you: your daughter is likely to share at least a few axes of any rich, white, able-bodied, upper-class etc privilege you have and further cannot serve to represent all women's experiences to you.