What is the definition of Sikh? As someone stated that a non sikh got married to a sikh and does alot of seva and paath, but the reply was that she shouldn't have got married in a gurdwara as she has not converted. What does conversion mean, just changing their surname? What about the so called sikhs who don't do paath or seva are allowed to get married in the gurdwara just because ''their kids have a chance of becoming sikhs''.
Unless there is one rule for all, we can't judge someone.
Also for your information as far as I know, many itihaasik (historical) gurdwaras in India do not allow any Sikh whose not sabat surat to get married in the Gurdwara, if we want to stop non sikhs from getting married, we need to do the same with people from our own community as by being born into a Sikh family doesn't automatically makes you qualified.