Even gibbons are monogamous

I went to the National Zoo today. On the way rushing back to the exit, we managed to stop by at the apes area and just happened to pay a bit more attention on this guy: ungka kelabu (Bornean Gibbon).

Looks like a loyal creature, isn't he? You bet he is. The information board says that these gibbons are monogamous and that they mate for life.

And a bit closer...

Enough with the big hoo-haa AA Gym created several months ago on monogamy and polygamy.

But what's going on here? Monogamous apes? For the world's sake, animals don't register their marriages. Is this the true loyalty, or monkey love? Or perhaps they all look similar - that it leaves no motivation or desire to find the second partner...