Are marital relations not supposed to pleasurable because they are only for reproductive purposes?
The primary purposes of marital relations are procreation and spousal unity (babies and bonding), but that does not mean that the husband and wife are not supposed to derive pleasure from their marital relations. Although pleasure is not a purpose of sex, it is important to fulfilling the purposes of sex.
God gave humans the capacity to enjoy food and sex in order to encourage them to fulfill the purposes of those human needs. A moral problem occurs only when deriving pleasure from food and sex is put above or in the place of its intended purpose. Someone who eats solely for pleasure, without regard for the needs of his body, abuses the privilege of eating. In the same way, someone who has sex solely for pleasure, without regard for its intended purposes of babies and bonding, abuses the privilege of sex.