Punjabi is an ethnicity and you (or anyone else for that matter) accepting it or not won't change this FACT
Punjabi is a linguistic identity, which includes many sub-groups like Hazara (mixed Punjabi-Pukhtoons,) Seraiki (South Punjab,) Pahari (North Punjab.)
Even in Central Punjab, one of the most prevalent groups, Niazis, are Pukhtoon origin Punjabis (Imran Khan is one of them.)
Around Lahore and the surrounding areas, the two main ethnic groups are Arain (Mian, Malik,etc.) and Rajputs (along with Jats, Gujjars, Bhattis, Bajwas) have completely different origins.
And no matter how much you want to deny it the fact remains that Pakistani Punjabis share common ancestry, ethnicity, language, culture etc with Indian Punjabis
We have diverged a long time ago and Sikhs have different tribes, ethnic groups which Punjabi Musulman do not have. Racially, Pakistani Punjabis have mixed with Pukhtoons, Kashmiris, Sindhis, Baloch, and other groups for almost one thousand years, while Indian Sikh Punjabis have not, as most of them have Hindu origins.
A Punjabi marrying Non-Punjabi in Pakistan is an exception (that too limited to large urban centers only), and not a norm
It is very common now a days. Pakistanis are Pakistanis, regardless of linguistic or regional origins. As Pakistani nationalism becomes the norm, you will see more and more marriages like these.
In my family, we have married into almost all provinces in Pakistan. Half of my family are married to Pukhtoons.