Oh, I am sorry that I did not refer to which sentence I actually was relating my answer.

It was this:

I must say that I have never thought of getting married myself, although I also don't dismiss the idea at all.

There are always exceptions to a rule. If official marriage is the rule but there are people who live together in true love without having married officially, they actually are married unofficially (because they know the rules by heart from what they grasped from the idealism of religion).

You asked

Does that mean that one must be responsible for the feelings of others that we provoke?

In the context of this post and what Chesterton wrote, I interpret him like so:

If you do something with pathos, i.e. provoke the passion of a woman or a man who is in a committed relationship, there is a responsibility on the part of the initiator. Of course, he shares this responsibility with the person who responds to the seduction, but it is not diminished by this.

For example, if a seducer, as Casanova became known for, tried to seduce a married woman by every trick in the book because she is married, then his own temptation lies in getting a married woman to commit adultery, for example. So it was something of a sport for him to get those women into his bed who were officially married wives. After he succeeded he lost immediate interest.

For a couple who feel that they are married (without having performed the official act of marriage before the church or registry office), it matters less whether the union is legal or not because they believe in the sacrament of the union. However, an outsider who is after the woman could say that it would be no big deal if he were to seduce her, as she is not married.

I think that might be what Chesterton could have meant with that sentence.