The game really is designed to be played with friends anyway (though whether there's more or less awkward than your character's romance with strangers is debatable).
Well, yes and no I'd say. IIRC there was an interview in which Larian admitted that 85-90% of all people who beought DOS only played the game in SP.
So designing the game exclusively for co-op with SP being a big afterthought would imo be a HUGE mistake given the simple fact that the overwhelming majority of their fans and actual players want a nice SP experience.
I can only suggest that Larian concentrates on the SP at least as much as on the MP because in the end, that's what a lot of people are interested in, with many of them never even touching the MP aspect.
Based on that line of thought I think that adding specific SP elements to the game would be a good and very appreciated idea, having deeper relationships between the player and his companions being among that. We have competitive question for co-op and that's awesome - for people who want to play the game in co-op. For the 85-90% of gamers who are interested in the SP there should be elements that make up for that. A serious and carefully implemented relationship system (which could include romance) would imo be a very fitting system that pretty much only works in SP. There is nothing wrong with that.
If you ask me, DOS 2 can be as much a SP game as it can be a co-op/MP game - if Larian only wants it to be that way. If they concentrate too much on the latter aspect they will alienate a lot of their fans though, by the way the same people they tried to attract with all the talk about the more serious writing, the inclusion of people like MCA and so on. They claimed that DOS 2 is meant to become their BG 2 - and honestly, that rises a lot of expectations for a lot of people.
Bottom line, I'm all in for a proper relationship system that includes shades of love and hate, if it's properly made (and I trust Larian enough to think that they are able to do so), even if it's a SP-only feature.