It ain't forced, you just want to look at them that way. The reason why PI and HQ click together is because HQ is easygoing while also being damaged so there's this side of her that compliments with Ivy's reserved and seductive side of her. Ivy sees in Harley someone she can trust and have her back when she needs someone on her side, Harley sees in Ivy someone that will always be with her no-matter-what, someone that won't belittle her and always see the good in her. The two formed a friendship that was based on mutual trust, it was only natural that it grew something beyond that. Harley also has in Ivy someone that always looked for her best interest and her well-being by trying to separate her from Joker.
The two aren't forced, it just a natural friendship that grew up to be a romance.