Firstly: 

It is obligatory for both spouses to keep the secrets of the marriage, especially anything that has to do with their intimate relationship in bed.  The wife is entrusted with the husband’s secrets and the husband is entrusted with the wife’s secrets. 

It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) turned to the men and said, “Is there any man among you who, when he comes to his wife, he locks the door, throws his blanket over himself and conceals himself with the cover of Allaah?” They said, “Yes.” He said, “And does he sit after that and say, ‘I did such and such, and I did such and such?’” They remained silent. Then he turned to the women and said, “Is there anyone among you who speaks (of private marital matters)?” They remained silent. Then a buxom young girl sat up tall so that the Messenger of Allaah (peace and blessings of Allaah be upon him) would see her and hear her words, and said, “O Messenger of Allaah, (the men) speak and (the women) speak.” He said, “Do you know what the likeness of that is? The likeness of that is that of a female devil who meets a male devil in the street and he fulfils his desire with her when the people are looking on.” 

Narrated by Abu Dawood, 2174. Classed as saheeh by Shaykh al-Albaani in Saheeh al-Jaami’, 7037. 

Secondly: 

With regard to your husband’s other marriages, if that is for the purpose of “having a change” as you say, then this is marriage with the intention of divorcing, which is deceiving the woman and her guardians (walis). 

Shaykh Muhammad Rasheed Rida (may Allaah have mercy on him) said: 

The fact that the scholars of the earlier and later generations emphatically forbade mut’ah (temporary marriage) implies that marriage with the intention of divorce is haraam, even though the fuqaha’ said that a marriage contract is valid if the husband intends it to be temporary but did not state that as a condition in the marriage contract; but his concealing that is regarded as a betrayal and deceit, and this contract deserves to be annulled more than one in which he stipulated the condition that it be temporary with the agreement of the husband, the wife and the wife’s guardian. This leads to many evil consequences as it is abusing this great bond which is the greatest of human relationships, and going along with one’s whims and desires. When this condition is not stated clearly, that is cheating and betrayal which leads to other bad consequences such as enmity, hatred and loss of trust even of sincere people who want to get married in the real sense, which means protecting the chastity of both partners and cooperating in establishing a righteous home… 

Quoted from Fiqh al-Sunnah by al-Sayyid al-Saabiq, 2/39 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) spoke similar words on the prohibition of such marriages. He said: 

Moreover this view – the view that (such marriages) are permitted – may be exploited by those who are weak in faith for evil purposes, as we hear that some people have started to go on vacation, during time off from studies, to other countries to get married with the intention of divorcing soon. I have been told that some of them marry several wives on these trips, as if they go there only to satisfy their desires which are more like zina (adultery, fornication) – we seek refuge with Allaah.

Because of this we think that even if there is a view that it is permitted, it is not appropriate to open this door because it has becomes a means that leads to that which we have mentioned. 

With regard to my opinion on this matter, I say that this marriage contract is a valid contract, but it involves deceit and betrayal, so it may become haraam because of that. 

It is deceit and betrayal because the wife and her guardian, if they knew the intention of this husband, and that his intention is to enjoy intimacy with her and then divorce her, they would not adept this marriage. So in that sense he is deceiving and betraying them. 

If he tells them that he wants her to stay with him for the duration of his visit to that country, and they agree to that, then this marriage is mut’ah (temporary marriage). 

Hence I think that it is haraam, but if anyone goes ahead and does it, then the marriage contract is valid, but it involves sin. 

Liqa’ al-Baab al-Maftooh, Question 1391. 

But if he married you with the intention of making the marriage permanent, and he has no intention of divorce, but then something happens which is a reason for divorce, then there is no sin on him in that case. 

Thirdly: 

With regard to his getting married in secret, if that is in the presence of the woman’s guardian and two witnesses, and the marriage contract is done in that fashion, then it is a valid marriage. But if it is done without the woman’s guardian or without the presence of two witnesses, then the marriage is not valid. See questions no. 7989 and 2127. 

Fourthly: 

We advise your husband to fear Allaah with regard to his family, and to fear Him with regard to people’s honour. He should note that it is not permissible for him to fool about in this manner, for marriage is love, tranquility and compassion, so he should not make it merely a means of satisfying his desire and then abandon the woman.

We also advise you to be gentle in denouncing your husband, and to preserve the stability of the home, and be sure of what you mention about his intention in taking several wives and what you do not like about him. Remember that a woman’s jealousy towards co-wives may lead her to make mountains out of molehills, and the waswaas (whispers) of the Shaytaan may make it worse, with the aim of disrupting the stability of Muslim families. 

So look at the matter with a little wisdom, especially the matter of his intention, which you cannot really know. Ask Allaah to show you the truth of the matter, and pray istikhaarah with regard to either staying with him or asking for separation. Think about your situation if you get divorced and what the consequences of that would be, then decide whether it is better for you to leave him or to stay and put up with it.  If you cannot bear it because of the things you have mentioned, then you have the right to ask for separation. 

And Allaah knows best.