If JOSEPH were NOT planning to consummate his marriage to Mary, why did he plan to DIVORCE her?
(Ain't speckelatin' FUN!!??)
Perhaps Joseph also wanted to focus his attention on God? Why would this be a strange concept to you?
Well, maybe you are not a Christian, maybe you are an atheist, and I had misunderstood what you are doing here.
As to why he would want to divorce her, think about it. The two have a plan to live a celibate life together, foregoing all the priveleges of married life, and then she turns up pregnant, which is generally an indication of sexual activity. She seems not to be the sort of person she had shown herself to be. Of course Joseph would divorce her, to allow her to marry the father of her child, or someone who did not want a celibate marriage as he seems to have done.