There is a reason for fertility clinics to (pay) men (some) money to go into a private room to watch videos or porn to jack off, then store it safely with your identity (private) to match up to any potential clients for a child.
Now in this case, many times the woman may have to be inseminated many times with the hopes of being pregnant and believe me the cost can be prohibited since insurance does not cover this. My step daughter tried this and it cost her over $10,000 and she gave up.
But in your case, they will be paying you (unknown) amount which by doing the “natural” way they hope to get lucky (maybe) in a month or two. But what if, the spouse gets angry or upset that you are doing this, even though they say it is okay. It is one thing to say ok to screw my spouse, but another thing when they think about it later, or if you are friends with this couple, may not want not want to associate with you (at all) due to this later.
Or, what (if) before the lady gives birth and they break up, then she can go after you for child support for the next 18 years, since it will be your sperm that will show that you are the “natural” father.
My suggestion is to say thanks, but no thanks and let them find someone else.