If you are religious and your son or daughter wants to spend his/her life with an atheist, what would you do? If you are an atheist and your son or daughter decides to spend his/her life with a religious person, what would you do?
Respect their choice, so long as the religion or lack thereof was not abusive.
For example if my child were an atheist and the religious person in question they wanted to spend their life with was the kind who looked down upon others for not being with their religion, the kind who constantly tried to subject others to their religion and say that they were going to some bad place for not believing, the kind who would ban various subjects such as books or television because they thought it to be the work of some evil entity that is sinful to view or anything else along those lines then, and only then, would I have a problem.
If my child grew up religious and wanted to spend life with an atheist as long as that atheist respected their religious views and the differences in the fact wouldn't get in the way of their relationship I wouldn't care. But if that atheist was trying to get my child to be atheist as well or using whatever to try and make them feel bad for their religious choices then we'll have an issue.