Instead of using the above as reason to have sex before marriage, perhaps you will get a better response if you pose it as a problem with waiting for marriage and then ask what can be done to fix it, thus leading more to consider the issues instead of just writing you off as someone who is sexually immoral. And to make it better, perhaps couple it with the verses that say it is a husband's duty to fulfill his wife's sexual needs and a wife's duty to do the same for her husband.
Um thanks...?
If somebody wants to know if I'm sexually immoral, they can ask what my sexual values are.
If they write me off before that, then why would I care about their opinion any more than they do? (they obviously don't, if they don't care if it's informed or not).
If they write me off afterward, that's their right but I'd be really concerned about
their sexual morality, in that case.
Ultimately, though--while there are things that can kinda-sorta
help, like talking about fantasies and values--my main point is that thinking about sex is different than having it, and compatability should be established before marriage.
I could try to set it out that way, but it would look...odd.
Problem: sexual compatibility is a majorly important part of a marriage, and there is no way to establish that it exists without actually having sex. You can roll dice about how your sex life will turn out, or you can go into marriage informed.
Solution: the only solution is to have sexual experiences with each other, prior to getting married. Not necessarily every sex act you might ever want, but enough to get a sense for each other.
If you really, really don't want to do that, talking about hypotheticals can give you some idea about the other person's values and interests, but it's a poor substitute, given that so much of what happens during sex can't be easily predicted.
edit: I
could use the Bible to back up my answer...and you could use the Vedas...but why would either of us do that?