I think the "Opie Loves Helen" episode is quite romantic. We see Opie buying a gift (stockings!) for his new crush; Andy teases Barney by asking him when he's going to marry Thelma Lou; Barney flirts on the telephone; and Barney describes a past romantic date with Thelma Lou. There's a lot of romantic fun in this episode!
Barney telling Andy all the things he'd like to tell Thelma Lou if he wasn't so shy, is very romantic.