There’s a lot of history and bigotry over the last 500 or so years towards and surrounding specifically Ashkenazi Jews that’s lead to the current day feelings about Jews marrying non Jews. It’s kind of silly, it’ll change over time if Jews are integrated more with society. American Jews aren’t putting as much emphasis on it. Sephardic Jews by comparison are much less worried about it, and most aren’t “purebred” in the same way. There was a lot more intermingling between the Muslims, Jews, and Christian’s at the time in those countries. What it really comes down to is tradition and your community. Different Jewish communities will have emphases on different things. I haven’t done a deep dive on this Redditor but it sounds like his partners family insists on him converting and is orthodox. He hasn’t mentioned if he’s kosher or if they are. Some orthodox believe that there is no real converting, and believe that if a convert breaks a single rule they aren’t really a convert. Chances are if they’re allowing their daughter to marry a convert they’re probably more progressive than their label would suggest.

What I can absolutely assure you is that converting is no joke, and if you don’t want it you can’t do it. You gotta want it, and are often refused the right to and have to “prove” your desire, as well as learn and study a ton of Torah. I wouldn’t think of it as this Redditor is being pressured to change, but more so that they want to be involved in their partners’ family’s traditions and culture, and most likely find it beautiful enough to want to join.