There's a number of reasons why slut-shaming may exist amongst guys whether it's to do with jealousy or if it's just simply to do with long-term suitability from a cost/benefit analysis. Some guys shame sluts because they pass over nice-guys (or use them) in favour of Chads or assholes, feeling dejected that they'll fuck everyone else except for them. Whilst others wouldn't want to subject themselves as the "beta-bux" whom she settled for rather than desired or that they're more prone to cheat, divorce-rape you etc due to their hypergamous instincts.

So to resolve this issue, the stigma against LTRing sluts would be abolished if:

-They were faithful.
-There's no such thing as alphafux/betabux - meaning that Chads and nice-guys were given equal shots via 'sexual socialism'.

For the former, I know not every slutty girl cheats so let's just assume for arguments sake that they're less likely to cheat. But as for the latter, no one likes to feel like they're second best or plan-B doing all the boring stuff and providing for with their ungrateful wives in a dead bedroom scenario. So provided that these criteria were followed in a perfect world, I'd absolutely have no problem LTRing a slut (even if her count was in the 100s). Otherwise, there are usually risks and nobody likes to feel like they're with someone who would've rejected them in their prime youth.