I agree with this, but I would rephrase it: one of the conditions of true love is acceptance of your partner, of all their flaws, mistakes, etc. Now, I am not one to talk on this particular subject, since I have... pretty unorthodox views on sex in relationships and outside of them. But even if I believed that sleeping with someone else was a horrible moral crime, I wouldn't dismiss someone I truly love just because they did it. When two people truly love each other and get very close to each other, they are almost guaranteed to cause each other a lot of pain, along with pleasure, since the strongest attachments cause the strongest feelings, both positive and negative. Being able to handle this pain, to get over it, to accept your partner's mistakes and to forgive them for causing pain to you - I think this is the road to truly fulfilling relationships.
Of course, there is a limit to it. No matter how much I love someone, I probably wouldn't be able to accept them if I learned that they worked as killers on the side, for example. At some point, the overwhelming pain cuts off the attachment. Is your partner cheating on you with someone once or twice enough for that to happen? I think it depends solely on your personality. to me, it wouldn't be such a big deal as to most other people.