@Canisa mentions what is essentially the heart of the argument (and it’s the point that I think men often times miss, or misunderstand). The sexuality of a character is not the issue. In fact, the sexuality of the character is, essentially, a complete non-issue. It’s not their sexuality of a character, it’s how their sexuality is treated fundamentally differently from other characters and used to define them, and, in addition to the more commonly recognized, removing the character from any central importance of the story, and regulating them to a sex object.
Let’s take a character to use as an example. How about Major Kusanagi Motoko (specifically focusing on the anime)? (I picked her because I think we have a lot anime lovers here, and for those that don’t know her, there’s a plethora of readily available information, plus she’s a character I’m very familiar with.) She’s the focus of the series, plus she’s faster, stronger, smarter, more determined, and arguably ‘deeper’ than any other character in the stories. Clearly this makes her a strong female character, right? Well, what do you remember about her besides those things? Maybe her outfit in the first season, that skimpy ‘leotard’? How about what she did? How often did she play a honeypot? What about her, ah, preference in ‘company’, namely her ‘sleepovers’ with other women? While there are legitimately gay character, was her portrayal in such situations anything more than fan service? While these things don’t destroy her otherwise strong characterization, they function to weaken it. They are unnecessary, and therefore ‘over’ sexualization. (The second season, IIRC tones down a few of these things, most notably in that her outfit, while still worn a little more sexually than the men’s, is ultimately the same uniform. Did anyone look at that and complain that’s ‘shes not the same’? Sure, some people probably complained that she wasn’t as ‘sexy’, but did anyone say ‘she’s less believable now’? What about if her outfit change had gone in the other direction? And IIRC Batou get a little more sexualized with tighter tank tops and generally more focus on his body, although I think it was mostly used in highlighting the difference between him and Motoko.)
How about we take another character? We’ll use a Lifetime Movie heroine (it doesn’t matter which Lifetime Movie, the vast majority of them are ultimately the same story). She’s single (usually divorced), struggles to make ends meet, has a child (or, rarely, another type of ward) and she has a problem. She struggles with her problem, and eventually overcomes it. To top it off, the movies are aimed at women, so she’s rarely (if ever) highly sexualized (although she always looks good). That’s a perfect formula for a good strong female character, right? Well, let’s take a closer look at the average Lifetime Movie plot. First, there are a few scenes which set up the woman as being a struggling single mom/whatever characteristic she has that you’re supposed to identify with. Then she meets a man. Then, if not already brought up, we see what difficulty she will face the movie. If it has not already been brought out, we then find out that the man works in a field that lets him help to fix her problem. Then the two of them have a short ‘will they or won’t they’ bit before getting together, usually with the woman leaving and the man chasing her. Finally, soon (or immediately) after they get together, she has her final confrontation with the main antagonist (who is virtually always male). In this confrontation she finds that she is unable to beat the antagonist until the man comes and rescues her. What’s this sound like to you? Man meets woman, man loses woman, man finds woman, man fights for woman. This is simply the standard hetero-normative story, albeit with the possibility of a few small quirks, from the female perspective. (My wife mentioned that they are slowly starting to drift away from this formula a little, but still, you see the pattern laid out.)
These two examples are (IMO) the two largest, most prevalent, and easiest traps to fall into when attempting to make a strong female character. The first is a strong character, weakened by reducing her to simple fanservice. The second is a character who may at first appear strong, but ultimately plays second fiddle to a male character.
(Quick Clarification: I’m using the term ‘sexuality’ above with the definition of ‘the state or quality of being sexual’, not the more commonly used definition of ‘sexual orientation’)