IMVU is sort of a virtual world that was established in 2004 and was very popular at that time. It allows you to create 3D avatars in a fun and safe virtual world and that too for free. It is basically a multiplayer online video game that brings people together on an online platform

Imvu is best no doubt but if u wanna explore more , u can try these games:
1. Second life:Second Life is an amazing IMVU alternative which is also available for free. The game is created by Linden LAB and is available for PC and MAC since 2003. This game is perfect for teenagers and has numerous customization options and possibilities. The good thing is that you can also make real money by selling the game currency to anyone else. The game has people from all around the world which allow you to meet different people and make new friends.

The game has a vast virtual world which allows the players to explore it in detail and interact with each other. You can buy a lot of things in the game and trade the products with other players or users in the game. There are many social activities too such as clubs and gatherings that will give you a real life joy while you’re playing. The game currency is called ‘Linden dollar’ which can be used for making transactions and also convert it into dollars to earn some money and have fun at the same time. Its as wild and naughty as imvu *winks*
2. SIMs city. Its not a multi player but still its good and i preferable like it

Many years ago, before I joined IMVU, I had already tried SecondLife for a few days...I don't know why I was never really convinced, or at least I never found the right motivation to dwell on SecondLife.
Over the years I've tried several times to return to SecondLife, to this day I still have installed one of the third-party clients that I used for a brief period when I frequented SecondLife more assiduously.

If there's something I've always appreciated about SecondLife, it's the fervent (at least until a few years ago) SecondLife community, which, thanks in part to the type of license used by the official client, allowed the development of numerous third-party clients. Not to mention then, of course, the native cross-platform support of the official client and many third-party clients.
I think it's also worth mentioning among others the OpenSimulator project, based on the SecondLife code, which is currently still used by OSGrid, DreamGrid and Diva Distro.

Of course, there are many other projects that can be likened to SecondLife and IMVU.
I had a chance to try some of them and I uninstalled some of them after a few seconds after installation, while others I tested a little more thoroughly.

Of course, the best experience I had was on IMVU but I think the credit for that was some of the people I met and hung out with there.
SecondLife remains a historic project and for some features even far ahead of IMVU, but I think the credit is due to the various communities that have helped to improve it.

I currently prefer other projects, I don't know how good or better they really are compared to SecondLife and IMVU, but at the moment I feel more comfortable attending these rather than SL or IMVU.

Unfortunately there are not many games or MMOs with such good quality. I can recommend the following:
Yareel online, is similar to imvu but focused 100 to sexual interactivos.
It has good animations but is loaded with micropayments and customization is very limited
MNF CLUB: It's a 2d sex mmo, maybe you know the Meet and Fuck games, it's from the same developers. It has good animations and art, the bad thing is the payments to be able to have good poses, sometimes they are excessive and also its community is too toxic. It has its good stuff, good customization of characters and rooms, smooth animations, also a very large community, but like I said, it's very toxic. You could take a look at those two.

How's the adult element on SecondLife? Is it restrictive like IMVU where you gotta find around ways to get nude outfits and dick triggers like black markets for vu, or is nudity and sex a part of the game?
Also, how's the game's mobile port? cuz I do prefer mobile for all the sexy stuff over the pc clients ;)

there is a free game called meridian 3dxchat its a copy of the paid version 3dxchat.

There is also Match D which is the same as Meredian, both are copies of the paid 3DXChat, Match 3D has fewer users than Meredian, but it has more avatar customization options, including a nude option.
The poses and graphics are much better than imvu, there the focus is on sex, orgies in public rooms, there aren't many groups of friends, there are some, but it's not like imvu.

NOTE:
I use a translator, please disregard some errors.