Note that I do not say "strong" female characters. In most cases, strong is a misnomer. it gets you thinking the wrong way. Strong usually means that you take a character that's a male and switch gender, so the character looks female, but in actuality, she's just a male character with a different skin. Even worse, she's most likely a shallow, poorly writtten male character with a female skin. Your goal is to write a female character, which means taking advantage of the fact that she's a woman.
If you only have one female character, you'll sink her into a rut of stereotypiness. The more female characters that you can have in your story, the more that you'll prevent yourself from falling into stereotypiness by having variety. More women forces variety on you. You usually can't have two dull women characters at the same time. The characters will natually differentiate themselves.
For example, we have two warrior sisters. They're both extreme, tough, and beat up anyone in their way, acting in all ways interchangeable. But one is blonde while the other is brunette, so that's OK? No. Hair color doesn't matter. On the other hand, if you have two warrior sisters, one which thinks tactically, always reviewing their past battles, while the other one is all about pressing the advantage, then you have two different approaches to the same problem, which means that these characters will act and think differently about the same situation. You also have a built in conflict between them, not because you intended the conflict to be there, but merely because their styles will eventually put them at odds.
What you want to avoid is sex while you develop your characters. An interesting character without sex is interesting, but a sexy character without interest is boring. Make your character interesting before throwing sex into the mix. From our above example, you can see that the pensive one may wait for the interested suitor to approach, while the aggressive one may wade into her pool of potential mates. These relationship will be interesting because their approaches are interesting. So the more that you develop them before you think about sex, the better that the sex/romance will turn out.
And then there are different women in different parts of their life. Just the perspective of older and younger can greatly change a character's outlook.
Those warrior sisters have a mother who still worries about them, teleporting in and saving them far too often. Let's just say that life with mom gets a bit tough when mom is so overprotective. And mom has her opinions, and she certainly lets you know about them, especially when it comes to men. It's not that she's cruel or a man-hater or anything, but she is a shrewd judge of quality and making a profitable match. That, and she really wants grandchildren sooner than now. It's a mother thing.
Meanwhile, younger sister is busting her britches to go on an adventure and she just isn't ready. She'll all ready to prove herself without realizing just how unready she is.
Note that I never used the word "strong" to describe any of these characters. I didn't have to.