@ John: There is nothing wrong with merely being interested in more than one NPC (i.e. Stage 1) However, as soon as you move to Stage 2, the NPC will have a vested interest in you.
For example, let's say that you get to Stage 2 with Cleo and you get the ability that allows you to summon her to your side. In the mean time, let's say you have also hit Stage 2 with someone like Serenity. Then you would be taking the "Cassanova" route. While this is totally do-able, it does mean that when one of them finds out, the excrement will hit the fan in an awe-inspiring manner. Basically, once you surpass the "I like you" stage and actively decide to pursue the romance, you're locked in with that character. If you choose to abandon the relationship, you abandon any perks that it provides.
Let's say you get to Level 3 with both Serenity and Cleo. You will enjoy the benefits of both (at least temporarily.) Unfortunately, you will have to make a choice between the two, and when you do, you will lose ALL benefits from the party that you abandoned.
Btw, as of right now, I have 7 possible romance options available for NPCs. However, if you stumble upon an NPC that is not named as a potential love interest, I can likely convert that for you.
Almost forgot... It is entirely possible to pursue a romance and still keep everyone else as allies. For example, let's say you pursue Serenity as a romance option, but keep Cleo as an ally. Cleo will still provide you with at least one spell, skill, or attribute, depending upon the level of your friendship.