Why is sex such a big issue and sin?

My point is this sex is for procreation 'thats the aim anyway' during marriage only otherwise it's a sin, why is it such a sin if two people love each other to make love? when we die you can't have it in heaven and jesus said man and wife will be no more as man and wife but as brother and sister, there is no marriage in heaven 'not even for the angels, do you think they miss it?' why is it this way what's more natural then a man to make love to his wife? okay some answers will be where are spirits and not physical bodies, why give us something so natural and great 'granted in some cases not so great' we make it to heaven and then take away something so natural to show our love to our soulmate? I'm talking about the love in our loves 'soulmates' not booty calls, Why is is such an issue?