There are many physical, emotional and hormonal changes that take place in your partner's body during pregnancy which will influence how she feels.

There is no reason why you should not have sex during the pregnancy unless you have been advised to abstain.
The Prostaglandin hormone in semen can irritate the neck of the womb, and may start contractions so towards the end of the pregnancy it may be advisable to use a condom, unless you are trying to get labour started.