I found several forum posts, but the only solution seems to be to temporarily turn off iMessage, which seems like a terrible solution because it means that you won’t be able to receive iMessages from anyone else in the interim. Worse, the iMessages will look to the sender like they got delivered because they’ll still go to your Mac or iPad.
Ideally, there would be a way to simply start a new conversation using SMS even though there is an iMessage account associated with that phone number.
I absolutely agree. A text message is preferred when reception is spotty, and this whole thing gets very confusing when your talking to someone who has an iMessage account separate from their phone number and a phone number with SMS.