Adult content will offend some people.
Any decision to include the work in your portfolio should consider the fact that you may instantly lose a prospective client because you've offended them by showing the type of projects you'll work on.
Realize this isn't much different than if you created a web site for the Republican Party and a client is a Democrat. You can offend prospective clients that way as well. So it's not so much that it's "adult" oriented, it's that it's a divisive topic.
Obviously, if you are willing to do the work it doesn't offend you. There's no rule which states you must include these types of projects in a portfolio. You could do the work, then just not use it as a portfolio piece.
Be aware, there is a stigma much of the time... do one adult project, make it known, and you'll have a much harder time getting other projects. But you may find it easier to get more adult-themed projects. I'm not saying it's "right" or "fair" but often that one topic can do more to work against you than any other topic you may work on.
From what I understand (without any direct experience), you can still make a decent living by just working on adult-oriented projects. But much like being an adult performer, I hear it's very difficult to make the transition back to regular work after a period of adult-themed projects. Again, no direct experience with this, just what I've garnered from others.
For my business, I do my best to remain autonomous. I don't want to do any work which is overly opinionated and divisive. I'd really dislike it if I was negotiating with a $50k client and they stumbled upon some $1k web site I did 5 years ago that offends them enough to disregard me.