The people before us weren't perfect. Their mistakes are blueprints to learn from and build a better world
Who didn't have conservative social views back in the old days? It likely would have been exceptionally rare to find anyone who openly supported what we'd now call the LGBTQ+ community.
Also I'm pretty sure Marx & Lenin didn't write extensively on the topic of sexual orientation and gender identity because they weren't exactly hot button topics in the late 19th and early 20th centuries.
Can't change the past but we can be better now.