@williamming,
Not sure what country you are from, but in the US a person's parents cannot force them to marry someone. So as I see it, there are a couple possibilities going on here.
1. You are not really serious and this whole question is bogus. In that case, quit wasting people's time.
2. You are from some other country where traditionally parents arrange marriages in which there is not much you can do to alter the situation and going off with this girl could get you (or her) killed, especially if she ends up pregnant by you.
3. This is a legit situation in the US and the girl is too stupid or too timid to go against her parents. Or, the girl is all for her upcoming marriage and just wants a fling with someone. In either case I would stay away since she would still marry the guy and they could come back against you for child support even if the guy she is marrying is rich. You would be the one up a creek if she ends up pregnant.
4. She is hoping a fling with you will result in pregnancy and hubby to be will then not want her. Again, you end up screwed with having to pay child support for the next 18 years. Is that what you want? If not, stay away.