Fetus:
Not all animals have the limbic system and so not all animals enjoy sex. And those that have sex do not have it simply because it is pleasurable. If that were the case, most animal sex would not be limited to there productive cycles. Animals fight for, and have sex because it is an instinct. Pleasure, just as in humans I believe exist to increase the probability of success-- the exchange of sperm, and repetition.
For example, in humans, when the female orgasms, the vaginal walls are relaxed and sperm can permeate within that orifice with relative ease. The same processes exist with dogs and the other mammals--it is relaxing for the same purpose. One might be tempted to bring up homosexual sex and its enjoyment; but in truth, pleasure is derived by the same mechanics as those in heterosexual sex. Pleasures generated from spanking, pissing, etc are obviously mental.
To my knowledge, only Humans, Bonobos, and Dolphins engage in sex as a social activity-- for anything other than reproduction-- solely for pleasure: both of the event and socialization.
Eating as always been pleasurable even for a infant.
I disagree. Taste buds are an evolutionary trait. Food is only pleasurable with taste buds; this should be obvious. Prior to the acquisition of taste buds, pleasure in food could be equated to finally getting air after suffocating--relief or anything synonymous.