Assalaamu alaykum. if a guy knows that the girl he intended to marry has a liking for men with slightly long hair and he grows his hair longer so that she is more likely to accept him, is that ok?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

We have already highlighted that it is permissible for a man to keep his hair long as long as he does not imitate the non-Muslims, the dissolute people, or women, in which case it becomes forbidden. Please refer to fatwas 83385 and 84273.

There is nothing wrong with lengthening the hair in a permissible manner for a correct purpose such as encouraging a girl whom you want to propose to, to marry you, and so on.

Just as a man likes to see in a woman what will encourage him to marry her, it is the same with the woman.

Al-Mugheerah ibn Shu’bah, may Allah be pleased with him, narrated that he wanted to propose to a woman in marriage and the Prophet told him, “Go and look at her, for that is more likely to create love between the two of you.” [Ahmad, at-Tirmithi, An-Nasaa’i and Ibn Maajah]

As-San'aani said in Subul As-Salaam, “Such a ruling is also confirmed for a woman as she looks to the man who proposes to her, as she likes in him what he likes in her.”

Allah knows best.