How do you get two sims into a wholesome relationship?

I have no answer to this, but perhaps it has more to do with doing activities together and having a lot of (wholesome) fun - more this than all of the sexy stuff all of the time, if you know what I mean. 

I would try to make them cook together, watch movies together - go picknicking or something like that? Give eachother gifts and offer to bring them food at work when prompted. Feed eachother those chocolates. Things that are more sweet and affectionate, not necessarily lustful. 

https://www.ea.com/en/games/the-sims/the-sims-4/news/lovestruck-expansion-pack# 

Sounds like they need a good friendship level along with romantic relationship

I had to chuckle a little at the title, because my sims are allergic to anything that can be described as wholesome 

catloverplayerI kind of chuckled when I had a 'wholesome' romantic relationship popup in my game last night for this couple. Not sure what combo of triggers set it off, but the guy was their butler, plus I 'full-edit-mode' tweaked him a bit, so firing him or using him as a roommate was buggy for me (he kept switching back-and-forth to butler), so I finally had them ask him to join the household to fix it. Then I literally moved him into the bedroom of my target Sim to pair with him. However, they have only been on one date to get to know each other more, and I did it in the home. They have not kissed, and I don't have my Sims woo-hoo until they're married. Also, he's a 'high maintenance' Sim, and somewhat formal. I'm trying to have them 'court' in a modern way. So, I was not sure what all triggered that tag for the game, but it fit their relationship very well. I did choose all 5 types of romantic styles for both of them (gifts, etc.). Attaching screenshot of their details, in case it's of any use for this trigger/tag/question:

You can set the type of relationship in CAS as well. It's in the drop down menu where you set other types of relationships like family dynamics.

Same! When the game suggested this dynamic for one of my couples, I thought, what is it trying to tell me about my other couples' dynamics? 

I think you get wholesome dynamic when you don't have that many romantic interactions actually.  if they have too many, they get into steamy type relationship.