Marrying off orphans(male or /and females) if one cannnot keep them under own guardianship is one way to reduce ones resposibility which Allah SWT is putting on all in verse 4:2(they are like your family). The next "if you cannot do justice" is for when you are advised to marry females off when there are many you took responsibility of(in groups). Doing justice to all is not possible when there are number of them to take care of, even when one tries his/her best. So it best to take responsibility of just one or if one has already taken another person under his/her guardianship, this is enough!
In these verses Allah SWT is putting onus on the men and women of society, to take up resposibilty of the orphans in the society IN ANY CIRCUMSTANCES! Nothing of the sort of marrying to oneself is being said here... The verse does not imply you can marry them to your self or marry to someone already married or marry them before they reach the age of marraige OR marry them against their own wishes/likeness or if they dont want to in the first place (for both male or female orphans!) . All the rules regarding marraige in the whole Quran apply here too.
All that is being said here is that while marrying off many females one may not do justice. Sometimes just to get rid of that responsibility as soon as possible, or other time only because they are not their own flesh and blood.
And it is so very natural too. You might have observed in houses with lots of girls even their own parents are in hurry to marry them off sometimes not even thinking too much or finding about the person they are wedding them to, as females are regarded as financial burden. I have witnessed that twice in my family.
Allah SWT knows that this will happen and is not as Zaalim to let that poor female live with any unjust for ever that was caused even unintentionally.
So in order to protect those female orphans other verses are also talking about this unjust! Allah SWT is so Reheem, He also took care of this fact that even after marrying them off one cannot leave them on their own in case someone gets in trouble and telling us that even if one cannot do just with all that one took responsibility of, one cannot leave anyone hanging on their own thinking she's married already. So if some feels she is being not treated well by her husband one should do his/her duty of trying to resolve her problem not leaving her as orphan again!
Remember in case of the couple who fights but has families, arbitrators from both sides of the family are required. So for orphans this is being advised specifically for females as they are more vunerable to unjust!
Please read the words carefully again and again. Why Allah would say, "you cannot do justice with orphan women(not wives)...", and after confirming that it WILL happen go on saying, " and IF a married woman feels desertion from her husband ... "?