Well, I've never picked Isabela, so there's certainly precedent for that.
Don't get me wrong, I like Sera a lot. Hell, if possible, I'd enjoy a one true threesome. But if not that... the decision is tricky.
I'd still give other characters I have an interest in a go later down the road, still gonna pursue Josie and maybe Dorian. I'm still iffy about Cass, m/f romances tho everytime I'm struggling. 
Lels was a unique snowflake, I had no problems romancing her in various playthroughs but for DAI I have a few characters I like so I'm more eager to branch out this time round.
Keep Sera in mind though, she's a first for us. 