It's not about loose morals or a weak mind,
it's just that you may have been on the
single market for too long and have a
tendency of going into these types of
taboo relationships.
Julia Austin of Madame Noire lists these
kinds of forbidden love singles are likely to
engage in:
1. Step something: It could be as close as a
stepsibling, or a cousin by marriage, or
an awkwardly young step-uncle. But,
sharing so many similar experiences as
pseudo-family does, you can’t help but
feel a special bond with a certain
attractive non-blood relative at some
point.
2. A neighbour: Simply being exposed to
someone repeatedly can play a trick on
your mind, making you feel attracted to
that person. And the fact that your
neighbor is just down the hall or street,
so accessible when you’re feeling, um,
playful, can make this type of taboo love
become hot and heavy fast. But your
feelings can be confusing: is it proximity
or compatibility that’s keeping things
together?
3. A religiously committed person: Either
his family would disown him should he
marry a woman outside their religion, or
keeping up the religious tradition via
marriage/the bloodstream is genuinely
important to him—more important than
being with “the one.” Either way, you’ll
engage in a hot fling with a guy who
things really can’t go anywhere serious
with due to his religious background.
4. Lawyer/Client: A lawyer is almost like a
therapist—out of necessity, you tell him
every last detail of your life. You’ll
instantly feel bonded to him for this
reason, and, should he like the details of
your life, he may develop feelings for
you. Get a new lawyer, and get it on.
5. The renowned jerk: Everyone knows he
is a player/doesn’t respect women/
thinks he is God’s gift to the earth etc.
Nobody likes him. But you—well, you
yearn for him. And there’s something
hot about dating the bad boy.

God help us

I think its only no1 that is forbidden.
You should also include
A relationship with a someones wife or husband