I think it's fair to say that many, maybe most Americans employ some type of utilitarianism in their moral decision making. But there are at least two problems. One is that to judge the best outcome can be very subjective. What may be judged good for the pregnant woman may not be good for the baby. What may be judged good for the about-to-cheat-husband may not good for his wife or his children. This problem of subjectivity is inherent in utilitarianism for a second reason. Utilitarianism counsels that moral decisions should NOT be based on the inherent meaning of acts. Acts are only good or bad relative to outcomes. The natural law theory that I expounded in class assumes that human acts have an inherent meaning (remember my fist vs. extended hand of friendship example).
One of the most common applications of utilitarianism to sexual morality is the criterion of mutual consent. It is said that any sexual act is okay if the two or more people involved agree. Now no one can (or should) deny that for a sexual act to be moral there must be consent. Certainly, this is one reason why rape is morally wrong. But the question is whether this is enough.
If two men consent to engage in sexual acts, according to utilitarianism, such an act would be morally okay. But notice too that if a ten year old agrees to a sexual act with a 40 year old, such an act would also be moral if even it is illegal under the current law. Notice too that our concern is with morality, not law. So by the consent criterion, we would have to admit certain cases as moral which we presently would not approve of. The case of the 10 and 40 year olds might be excluded by adding a modification like "informed consent." Then as long as both parties agree with sufficient knowledge, the act would be morally okay. A little reflection would show, I think, that "informed consent" might be more difficult to apply in practice than in theory. But another problem would be where to draw the line between moral and immoral acts using only informed consent. For example, if a dog consents to engage in a sexual act with its human master, such an act would also be moral according to the consent criterion. If this impresses you as far-fetched, the point is not whether it might occur but by what criterion we could say that it is wrong. I don't think that it would be wrong according to the consent criterion.