Make no mistake about it, Pryor has a very poor record on LGBT rights. In a legal brief as state attorney general, he equated gay sex with pedophilia and bestiality and said LGBT people should be jailed because of it (details here and here).
As a judge, Pryor also upheld a law banning LGBT parents from adopting (details here). He was considered so anti-LGBT that in 2005, when George W. Bush nominated Pryor for a lower court judgeship, Lambda Legal said he was the "most demonstrably antigay judicial nominee in recent memory" (details here).
So while Trump himself may pretend personally to be pro-LGBT and once briefly held an upside-down rainbow flag, what he does as President will determine how pro- or anti-LGBT he actually is. Nominating Pryor would be a very dangerous move against LBGT and other civil rights -- his is stridently anti-choice, for instance.
Quite honestly, if an issue of marriage equality came before the Supreme Court again, Pryor could be the deciding vote to strike it down or limit it severely. He could also strike down other fundamental LGBT rights. After all, the high court has reversed its own precedent on a number of occasions in the past, and Pryor could be the vote to do that again and again and again, taking us back to the nineteenth century and earlier.