Are they big enough to warrant a market? Just as Bioware stated that gay relationship will never have more than straight relationship, like "putting the horse before the cart". I accepted that. Asexual is also a subset of either gay or straight, so the asexual romance will likely to replace the sexual romance from either one, and either groups will get upset because their sexual romance wasn't there. They tried it with Josephine, and that's why a lot of people are complaining about it. I personally would rather for them to make it an extra romance rather than counting it as part of the straight or gay romance if they really want to add the romance.
Well.. If you REALLY want to look into the science of it, and the cold hard statistics, then it goes something like this:
90% (at least) of the worlds population are heterosexual, thus comprises the vast majority obviously. The remaining 10% (or less) are divided amongst ALL other sexualities you can think of, of which homosexuality/bisexuality (some studies lump these two together, since they can be difficult to verify from eachother) are the largest at around 8% roughly. Leaving ALL other sexualities (bestiality, necrophelia, pedophelia, asexuality and a truck load of others) to the last 2% of the worlds population.
Now when you look at it from a percentage point of view, it may not seem like much, but the 10% "other than hetero" still numbers over 700 million people, which is a sizable market. So I'd say that it is very valuable to try to create content for them (the legal ones at least.........).
Now what they did with Josephine is kind of having your cake and eating it too. Since sexual relations or not directly shown but implied. So if you want to, you can head-canon that it didn't happen, or that it did if you want to. However, since some of the other romances directly show the sexual relationship, some of the Josephine romancers who wanted the sex, feels "cheated" of it.