It's not uncommon for folks working very closely together to form emotional & romantic relationships. (Aside: raising kids is the ultimate startup, that's kind of why marriage exists).
First, make sure you have clear founder agreements, focusing on the case where one of the founders leaves (this usually implies vesting of some form). If you enter a relationship and then break up, the odds of one of you leaving at some point is increased.
Second, if you have investors, a board, and/or other key stockholders, you should consider disclosing a serious relationship (should it get to that point, for some definition of "serious").
Some will say, "it's none of their business!" but it kind of is. Most companies of any size have a policy on workplace relationships, and the minimum is usually that they must be disclosed. Even if you're too small for an HR department, the issues around potential conflicts are the same.
You never, ever, ever want to be in a situation where you date, break up, one departs, the company is compromised, and an irate investor later says, "you never told me!".