There is a right answer. As some other commenters are saying, there is an objective right and wrong. If someone trusts and commits to you as fully as they do when they marry you (or common law marry by prolonged cohabitation), there is absolutely no excuse to damage that trust and that love for a selfish satisfaction of running a successful startup.

When you enter that type of relationship with someone, you commit, at the very least implicitly, to make your SO the primary priority in your life. If you want to run a startup, you have to work it around your relationship and not vice-versa. The amount of pain that's inflicted on many different people when a relationship that has all the trappings of permanence, lacking only the discipline of the participants to enforce that permanence, is extreme, and far worse and farther reaching than having a company fail or losing a bunch of investor money.

I understand that over the last several decades, individual "fulfillment" (aka undisciplined selfishness) has become the primary attainment for people, regardless of any social structure or moral definition. Marriage has been nullified and diminished to the point where most youth don't even see a point in it anymore. People have been taught, essentially, to worship themselves -- to put temporary individual interests above permanent social cohesion and the good of the whole, and really, the long-term interests of the individual as well. But that doesn't make this correct, and it definitely doesn't make a valid excuse to betray the trust of those closest to you -- triple especially if children are involved. True familial love is worth more than all the money in the world could purchase, including the money your startup may or may not generate, and this is an objective reality.