Is it okay if my wife has a lover just for sex and nothing else? How will that affect our marriage?

Let me try to break it down!

Is it ok?

It is really a subjective aspect and where you both draw your line of morality/limit in your relationship. As long as you both are ok with it and all is happening with consent of all involved, it is always ok.

How will that affect our marriage?

You cant really know how this will turn out.

It could really spice up things for you both in your sexual life and can also go otherwise. What is really import here is that you both have an open conversation with whatever you both are going through, physically as well as emotionally.

Most times, problem start to arise when one partner start to hide something with another and then complicacy starts to build up, which is obviously not going to be good for your marriage.

Keep and open conversation and create an environment to go with it together, it will definitely be amazing experience.

All the best!