Should two people only be engaging in sexual activity if they are trying to conceive a child?
No. But they should understand that it is a possibility and be prepared to be responsible for that possibility.
I don't think so but if you look at religious history - they don't really talk about married *fun time* - just premarital, homosexual, and create a child, sex. Which, in my mind, means it never was a problem that needed to be addressed.
Of course not.
Sex is the ultimate expression of love between partners. Not just baby making, otherwise it wouldn't feel so good.
There is quite a lot of sexual activity that can not result in conception, it would be a sin to rule that out.