God wants to love people, and he wants them to love him back. A person must be able to choose to love, or not love; or else the love isn't real.
If all God wanted were automatic worshippers, there would be no need for free will. But true love requires an opt-in contract.
Shouldn't we regard it as a Good Thing (tm) for God to give us the dignity of making a choice?
Why does God need to be worshipped? Isn't it said somewhere in the bible it's an offence to claim the mind of God, anyways?
First of all, the Bible does say God desires worship and love, so that isn't "claiming" anything.

Second of all, God doesn't "need" anything from us. He owns "the cattle on a thousand hills"; he created the universe; etc. He
wants to demonstrate love and grace and goodness, and the proper response to that is worship.
Marriage is not all about sex. Sex is (from a holistic viewpoint) only a fringe benefit. Marriage is all about a couple entering into a contract to love and care for one another. You're right; no marriage is guaranteed to (or is even inclined to) work, not automatically anyway. Marriage requires effort on both sides.
Of course sex is a good selection tool; if you plan to have sex with someone on a regular basis, it makes sense that you'd be more inclined to do it with someone you enjoy having sex with. But a marriage that's only about sex is a marriage that's only superficial.
Um, what's that got to do with my quote? Perhaps it'd be clearer if I said "why need marriage before sex?"
Actually, the biblical rule is that once you have sex with someone, you
are married to them -- two people becoming one flesh. The marriage "ceremony" is just a public declaration of your intent to enter into a marriage covenant. It's a celebration, which is good; and it's an invitation for friends and family to uphold the marriage, which is also good. But once you have sex, the covenant exists, whether or not you had a ceremony.
So taking this into consideration, "sex before marriage" isn't possible. If you have a bunch of partners, all you're basically doing is marrying and abandoning a whole bunch of people until you find one you like.
So, essentially, God says to kill unbelievers? (because it's not like God discourages conversion, after all.... who'd have thought a diety could be so hypocritical?)
He's not being hypocritical. These people were being wicked; they deserved judgement. So God judged them. God extends mercy in certain situations and in certain ways, but he is under no obligation to show mercy in particular circumstances. This was just one of those times.
He could have used a natural disaster as his instrument of punishment, as he has on many other occasions, but he wanted to demonstrate clearly to the neighboring nations that he was the one behind it all.
Right....by using men. That makes perfect sense.
Nooooo.....wait a sec. Men. Crikey. You could almost think - and I'm sure we've never seen this before - God was being used to justify the settling of old scores with extreme violence and cruelty.
Not really. The Israelites' quarrel was with the Egyptians; they held no particular animosity to the people in Canaan.
Look at the larger picture. The Israelites had just been divinely rescued through a great series of miracles, and news of their deliverance had spread all over the world. Now the Israelites were invading Canaan, also with God's help. God's objective was to demonstrate his role in both situations. The same God who delivered Israel from Egypt was also helping them invade Canaan.
Exactly. Doesn't make sense at all given the obviously non-monogamous nature of males; the whole 'spreading our seed' trait of our psychology which directly contradicts all this "marriage" bollocks. You'd have thought that God, given he designed us [rather badly, i'd say], would have either programmed out this trait or at the very least preached a polygamous marriage system.
Actually, nothing at all in the Bible prohibits polygamy. On the contrary, God expressly makes provisions for how husbands are to treat their wives, and lays down several rules. (For example, a man should not take the sister of a current wife as a new wife, because they'll become rivals.)
The requirement for a man to have only one wife at a time is a human invention, and a rather recent one at that. IIRC, it was part of Roman culture, and when the Roman Empire established Christianity as a state religion, it became part of Christian practice.