Does not taking a second wife (even after being able to treat wives fairly) fall under the category of imitating disbelievers? I heard that Abd ul aziz bin baz stated that in his book. Is it true? If a man is happy with one wife and doesn't want to take another (even after being financially stable to support second wife),then is he imitating disbelievers or going against sunnah?
I highly recommend you learn what it means Islamically when someone is imitating the disbeliever. This generally means when it comes to imitating their haram practices and the like. And in this situation, limiting yourself to a monogamous marriage is halal whether the disbelievers do it or not.
Not taking a second wife is not imitating the disbeliever's because marriage as an institution has always existed regardless of religion and place with very few exceptions. What Islam did was give and explicate marital rights to both husband and wife, and limit the amount of wives a man can have to 4. And there are Islamic scholars who prefer a man having 1 wife rather than multiple.
And if your fear that you will not do justice to the orphan, then, marry the women you like, in twos, in threes and in fours. But, if you fear that you will not maintain equity, then (keep to) one woman, or a bond-woman you own. It will be closer to your not doing injustice.
In this verse, the Qur’an neither establishes new permissions for polygyny nor encourages it, rather it sets limitations on its practice.
The Qur’anic exhortation to ‘marry only one’ is grounded in it being the most fair option to women and the preferred option as mentioned by classical jurists like Imam al-Shāfiʿī (d. 204 H).
And you shall never be able to maintain real equality between wives, even though you are eager to. So, do not lean totally (towards one) and leave the other hanging. And if you mend and fear Allah, then, Allah is Most-Forgiving, Very-Merciful.
Here, the Qur’an states, “You will never be able to treat your wives with equal fairness, however much you may desire to do so, but do not ignore one wife altogether, leaving her suspended…” which is often understood to be in conjunction with verse 3. The argument goes: if the requirement of polygyny is equal and fair treatment between wives, and this cannot be perfectly achieved, then indirectly the Qur’an is stating that monogamy is preferred.