However, it means there is now a part of the husband's heart the wife doesn't have access to or understand.
@julieb and others already the nail on the head about oneness, so I'll keep my mouth shut on that.
I just want to expand on the notion that one's heart is or isn't fully available to one's spouse. Aside from the excellent point about how the heart is capable of expanding to love more than one spouse just as one can love more than one child, it's important for us to recognize that the heart is not just controlled by an on/off switch or even by separate keys one uses to unlock particular chambers. The ability to open one's heart (and/or to love others and allow oneself to be loved by them) is something one can only develop over time and with great effort. We don't come out of the chute fully capable of globally loving anyone; it's much more of a growth process.
Therefore, it's a mistake to beat oneself up about any lack of "full access" to one's heart. As for the Facebook poster in question, it's therefore highly unlikely that his absolute whole heart would be entirely accessible whether he shared it with someone new or didn't share it with someone new. We all have things we hold back, even if we're unaware of it -- and I would contend that someone who didn't know how to be at least somewhat discriminating about what to keep to oneself would likely be hell-on-wheels as a partner.
I recommend avoiding the temptation to seek perfection in oneself. There are no perfect monogamists, so who among us has the right to expect that we will be perfect polygamists.
P.S. Please don't be sorry for inspiring this discussion. I'm grateful for you.