Like others, I'd recommend Josephine. It's very sweet, and has no sex scene, nor does it ever imply that they are having sex.
Iron Bull and Sera both have multiple sex scenes that are required to progress the relationship, so they are obviously out of the question. Bull's is also implied BDSM (although it is portrayed in a normal and healthy manner, not like that Fifty Shades of Grey garbage - thank you Bioware).
Cullen and Cassandra both only have one sex scene, but it is required. If you say no, the relationship ends.
Blackwall has a sex scene, but you can avoid it without ending the romance. However, I believe this prevents you from "committing" to the relationship.
Dorian has an optional sex scene. When he propositions you, you can ask him to slow down and he instead kisses you. It is, however, implied that they are having sex later in the game, regardless of what option you chose.
Solas has no sex scene, but he is still a pretty sexually forward character in his romantic affections. He also jokes about and implies that he is having sex with the Inquisitor.
I think Josephine is definitely the most "PG" of all the options.