"אישה שמסרבת או שאינה יכולה להביא ילדים לעולם, ולא מוכנה לתת גט לבעלה, מעכבת אותו מלבנות משפחה ולהמשיך את זרעו. במקרה שכזה הבעל רשאי לקחת לו פילגש ואין עם זה שום בעיה הלכתית. פסק ההלכה יסייע לאותם בעלים לקיים את מצוות המשכת זרעם גם במחיר של לקיחת פילגש על בסיס קבוע. אותה פילגש יכולה גם לגור יחד עם בני הזוג",
A woman who refuses or can not have children, and refuses to give her husband a get (divorce- BoT), delays her husband from creating a family and continuing his bloodline. In such a case, the husband may take a Pilegesh (Concumbine - BoT) and there is no halachic problem with it. This Psak will assist those husbands to fulfill the mitzva of procreation even at the cost of having a regular Pilegsh. This Pilegesh can also live together with the couple."
It is important in cases like this to be careful about interpreting the text of the answer. There is nothing here that would normally allow a husband to cheat on his wife. There are two conditions that must be fulfilled:
a. A wife who can't or won't have children with her husband.
b. A wife that won't give her husband a divorce (get).
It is slightly unclear from this short piece whether this includes cases where the wife can't have children, but the husband doesn't want a divorce. I tend to think it does because of the last line where he writes that the Pilegesh could even live with the couple. This also seems likely because the newspaper article further quotes Rav Abergil as saying that he instructed a Rosh Yeshivah of "a large Jerusalem Yeshiva" to act as such. I'll admit, I'm dying to know which Rosh Yeshiva lives at home with his wife and his pilegesh!
The question which is harder to answer is why would Rav Abergil allow someone to have a Pilegesh rather than just force the wife to give him a divorce?
See also this article in The Times Of Israel, which argues that:
This isn’t about concubines, but about marital freedom. Anyone who doesn’t believe spouses should be able to forcibly trap their partner in a marriage should support this ruling.