Good question. Like every organized religion on earth, Sikhism too has got some rules to be followed to be called a true Sikh. The Standard S.G.P.C Sikh Code of Conduct clearly says, that the persons professing faiths other than the Sikh faith cannot be joined in wedlock by the Anand Karaj ceremony and it also says, A Sikh's daughter must be married to a Sikh.
Not only the Sikh Code of Conduct but even the 52 Hukams of the Guru Gobind Singh Ji clearly says, Do not give your daughter's hand to a clean shaven.
So, the instructions to the Sikh community are very clear but unfortunately in the last 7 decades post India's independence some Sikhs have manipulated the Code of Conduct and start giving lame excuses like the one you mentioned above in your question. Some of the Sikhs have manipulated the things to such an extent that even those who are trimmers and clean shaven they too call themselves as GurSikhs. Our Gurus instructed us that Rehat or the Code of Conduct is more important (Rehat Pyari Mujhko Sikh Pyara Nahi)
I have heard some people saying that my ancestors were Sikhs or one of my parents is a Sikh or my wife is a Sikh so i am eligible to be called a half Sikh but the fact is in Sikhism there is no such term as half Sikh, quarter Sikh, 40% Sikh, 60% Sikh. In Sikhism either you are are a Sampooran Sikh following the Sikh Code of Conduct else you are not at all a Sikh.
Those who are unable to follow the Code of Conduct they do come up with marvelous excuses for example if someone is a non-vegetarian then he/she will manipulate the Gurbani and give an excuse as to why he/she is a non-vegetarian, if someone is a drunkard or trimmer or clean shaven then he too comes up with some fantastic manipulations, Oh! I am a Sikh at heart so what if i am not a Sabat Soorat but i do lots of Sewa and similarly if someone wants to marry a non-Sikh then she too comes up with some terrific ideas as to we all are one, we all are humans belonging to the human race so what's wrong if i marry a non-Sikh.
As i said above, like every organized religion on Earth, Sikhs too have their own Code of Conduct. To clear all the confusion that exists in the Sikh community, few years back, the Jathedar of the Akal Takht issued a Hukamnama instructing the Sikhs that Sikhs and non-Sikhs cannot be joined in the wedlock by the Anand Karaj procedures in a Gurdwara and lets not forget that the Hukamnama issued by the Jathedar of the Akal Takht (the Supreme Temporal Seat of the Sikhs) is equivalent to the Hukamnama issued by the Sikh Gurus.
