Letting people fall in love with people who are already married is one of the meaner things that God does...
Many people have asked why you would let yourself fall in love with someone who is married. That's a really stupid question. The obvious answer is that love, real love, doesn't take into account religion or race or marital status. Love is seldom convenient, and we sure as hell don't get to pick who we fall in love with. And unless it happens to you early in life, there's a good chance there will be other people involved...
I'm a romantic, and I truly believe that each one of us has a soul mate. However, there is this theory I call God's Law of Compensation. It works on the premise that each one of us gets a different set of goodies. Some people get looks, some get money, some get a perfect mate or fame or whatever. But no one gets everything. Perhaps God has decided that love is the one thing I should do without...